diff --git a/cmd/promtool/tsdb.go b/cmd/promtool/tsdb.go
index 91b97f5c5..1c6ae6f6b 100644
--- a/cmd/promtool/tsdb.go
+++ b/cmd/promtool/tsdb.go
@@ -472,8 +472,9 @@ func analyzeBlock(path, blockID string, limit int, runExtended bool) error {
 	}
 	lbls := labels.Labels{}
 	chks := []chunks.Meta{}
+	builder := labels.ScratchBuilder{}
 	for p.Next() {
-		if err = ir.Series(p.At(), &lbls, &chks); err != nil {
+		if err = ir.Series(p.At(), &builder, &lbls, &chks); err != nil {
 			return err
 		}
 		// Amount of the block time range not covered by this series.
@@ -589,10 +590,11 @@ func analyzeCompaction(block tsdb.BlockReader, indexr tsdb.IndexReader) (err err
 	nBuckets := 10
 	histogram := make([]int, nBuckets)
 	totalChunks := 0
+	var builder labels.ScratchBuilder
 	for postingsr.Next() {
 		lbsl := labels.Labels{}
 		var chks []chunks.Meta
-		if err := indexr.Series(postingsr.At(), &lbsl, &chks); err != nil {
+		if err := indexr.Series(postingsr.At(), &builder, &lbsl, &chks); err != nil {
 			return err
 		}
 
diff --git a/tsdb/block.go b/tsdb/block.go
index b06f0940a..33413bb21 100644
--- a/tsdb/block.go
+++ b/tsdb/block.go
@@ -82,7 +82,7 @@ type IndexReader interface {
 	// Series populates the given labels and chunk metas for the series identified
 	// by the reference.
 	// Returns storage.ErrNotFound if the ref does not resolve to a known series.
-	Series(ref storage.SeriesRef, lset *labels.Labels, chks *[]chunks.Meta) error
+	Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error
 
 	// LabelNames returns all the unique label names present in the index in sorted order.
 	LabelNames(matchers ...*labels.Matcher) ([]string, error)
@@ -499,8 +499,8 @@ func (r blockIndexReader) SortedPostings(p index.Postings) index.Postings {
 	return r.ir.SortedPostings(p)
 }
 
-func (r blockIndexReader) Series(ref storage.SeriesRef, lset *labels.Labels, chks *[]chunks.Meta) error {
-	if err := r.ir.Series(ref, lset, chks); err != nil {
+func (r blockIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
+	if err := r.ir.Series(ref, builder, lset, chks); err != nil {
 		return errors.Wrapf(err, "block: %s", r.b.Meta().ULID)
 	}
 	return nil
@@ -563,10 +563,11 @@ func (pb *Block) Delete(mint, maxt int64, ms ...*labels.Matcher) error {
 
 	var lset labels.Labels
 	var chks []chunks.Meta
+	var builder labels.ScratchBuilder
 
 Outer:
 	for p.Next() {
-		err := ir.Series(p.At(), &lset, &chks)
+		err := ir.Series(p.At(), &builder, &lset, &chks)
 		if err != nil {
 			return err
 		}
diff --git a/tsdb/db_test.go b/tsdb/db_test.go
index cea4b6e36..7eb3e272e 100644
--- a/tsdb/db_test.go
+++ b/tsdb/db_test.go
@@ -1830,6 +1830,8 @@ func TestChunkAtBlockBoundary(t *testing.T) {
 	err = db.Compact()
 	require.NoError(t, err)
 
+	var builder labels.ScratchBuilder
+
 	for _, block := range db.Blocks() {
 		r, err := block.Index()
 		require.NoError(t, err)
@@ -1849,7 +1851,7 @@ func TestChunkAtBlockBoundary(t *testing.T) {
 		chunkCount := 0
 
 		for p.Next() {
-			err = r.Series(p.At(), &lset, &chks)
+			err = r.Series(p.At(), &builder, &lset, &chks)
 			require.NoError(t, err)
 			for _, c := range chks {
 				require.True(t, meta.MinTime <= c.MinTime && c.MaxTime <= meta.MaxTime,
diff --git a/tsdb/head_read.go b/tsdb/head_read.go
index 985a15792..c6017e7b8 100644
--- a/tsdb/head_read.go
+++ b/tsdb/head_read.go
@@ -148,7 +148,7 @@ func (h *headIndexReader) SortedPostings(p index.Postings) index.Postings {
 }
 
 // Series returns the series for the given reference.
-func (h *headIndexReader) Series(ref storage.SeriesRef, lbls *labels.Labels, chks *[]chunks.Meta) error {
+func (h *headIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
 	s := h.head.series.getByID(chunks.HeadSeriesRef(ref))
 
 	if s == nil {
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index 59824ae08..f10dc9f64 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -1446,10 +1446,11 @@ func TestGCChunkAccess(t *testing.T) {
 
 	idx := h.indexRange(0, 1500)
 	var (
-		lset   labels.Labels
-		chunks []chunks.Meta
+		lset    labels.Labels
+		chunks  []chunks.Meta
+		builder labels.ScratchBuilder
 	)
-	require.NoError(t, idx.Series(1, &lset, &chunks))
+	require.NoError(t, idx.Series(1, &builder, &lset, &chunks))
 
 	require.Equal(t, labels.FromStrings("a", "1"), lset)
 	require.Equal(t, 2, len(chunks))
@@ -1499,10 +1500,11 @@ func TestGCSeriesAccess(t *testing.T) {
 
 	idx := h.indexRange(0, 2000)
 	var (
-		lset   labels.Labels
-		chunks []chunks.Meta
+		lset    labels.Labels
+		chunks  []chunks.Meta
+		builder labels.ScratchBuilder
 	)
-	require.NoError(t, idx.Series(1, &lset, &chunks))
+	require.NoError(t, idx.Series(1, &builder, &lset, &chunks))
 
 	require.Equal(t, labels.FromStrings("a", "1"), lset)
 	require.Equal(t, 2, len(chunks))
diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index ba78b5712..9cf2425ec 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -1597,7 +1597,7 @@ func (r *Reader) LabelValueFor(id storage.SeriesRef, label string) (string, erro
 }
 
 // Series reads the series with the given ID and writes its labels and chunks into lbls and chks.
-func (r *Reader) Series(id storage.SeriesRef, lbls *labels.Labels, chks *[]chunks.Meta) error {
+func (r *Reader) Series(id storage.SeriesRef, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
 	offset := id
 	// In version 2 series IDs are no longer exact references but series are 16-byte padded
 	// and the ID is the multiple of 16 of the actual position.
@@ -1608,7 +1608,7 @@ func (r *Reader) Series(id storage.SeriesRef, lbls *labels.Labels, chks *[]chunk
 	if d.Err() != nil {
 		return d.Err()
 	}
-	return errors.Wrap(r.dec.Series(d.Get(), lbls, chks), "read series")
+	return errors.Wrap(r.dec.Series(d.Get(), builder, lbls, chks), "read series")
 }
 
 func (r *Reader) Postings(name string, values ...string) (Postings, error) {
@@ -1836,8 +1836,9 @@ func (dec *Decoder) LabelValueFor(b []byte, label string) (string, error) {
 }
 
 // Series decodes a series entry from the given byte slice into lset and chks.
-func (dec *Decoder) Series(b []byte, lbls *labels.Labels, chks *[]chunks.Meta) error {
-	*lbls = (*lbls)[:0]
+// Previous contents of lbls can be overwritten - make sure you copy before retaining.
+func (dec *Decoder) Series(b []byte, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
+	builder.Reset()
 	*chks = (*chks)[:0]
 
 	d := encoding.Decbuf{B: b}
@@ -1861,8 +1862,9 @@ func (dec *Decoder) Series(b []byte, lbls *labels.Labels, chks *[]chunks.Meta) e
 			return errors.Wrap(err, "lookup label value")
 		}
 
-		*lbls = append(*lbls, labels.Label{Name: ln, Value: lv})
+		builder.Add(ln, lv)
 	}
+	builder.Overwrite(lbls)
 
 	// Read the chunks meta data.
 	k = d.Uvarint()
diff --git a/tsdb/index/index_test.go b/tsdb/index/index_test.go
index 6346f2ee8..ff22fb1c0 100644
--- a/tsdb/index/index_test.go
+++ b/tsdb/index/index_test.go
@@ -124,7 +124,7 @@ func (m mockIndex) SortedPostings(p Postings) Postings {
 	return NewListPostings(ep)
 }
 
-func (m mockIndex) Series(ref storage.SeriesRef, lset *labels.Labels, chks *[]chunks.Meta) error {
+func (m mockIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
 	s, ok := m.series[ref]
 	if !ok {
 		return errors.New("not found")
@@ -199,9 +199,10 @@ func TestIndexRW_Postings(t *testing.T) {
 
 	var l labels.Labels
 	var c []chunks.Meta
+	var builder labels.ScratchBuilder
 
 	for i := 0; p.Next(); i++ {
-		err := ir.Series(p.At(), &l, &c)
+		err := ir.Series(p.At(), &builder, &l, &c)
 
 		require.NoError(t, err)
 		require.Equal(t, 0, len(c))
@@ -311,6 +312,7 @@ func TestPostingsMany(t *testing.T) {
 		{in: []string{"126a", "126b", "127", "127a", "127b", "128", "128a", "128b", "129", "129a", "129b"}},
 	}
 
+	var builder labels.ScratchBuilder
 	for _, c := range cases {
 		it, err := ir.Postings("i", c.in...)
 		require.NoError(t, err)
@@ -319,7 +321,7 @@ func TestPostingsMany(t *testing.T) {
 		var lbls labels.Labels
 		var metas []chunks.Meta
 		for it.Next() {
-			require.NoError(t, ir.Series(it.At(), &lbls, &metas))
+			require.NoError(t, ir.Series(it.At(), &builder, &lbls, &metas))
 			got = append(got, lbls.Get("i"))
 		}
 		require.NoError(t, it.Err())
@@ -421,16 +423,17 @@ func TestPersistence_index_e2e(t *testing.T) {
 
 		var lset, explset labels.Labels
 		var chks, expchks []chunks.Meta
+		var builder labels.ScratchBuilder
 
 		for gotp.Next() {
 			require.True(t, expp.Next())
 
 			ref := gotp.At()
 
-			err := ir.Series(ref, &lset, &chks)
+			err := ir.Series(ref, &builder, &lset, &chks)
 			require.NoError(t, err)
 
-			err = mi.Series(expp.At(), &explset, &expchks)
+			err = mi.Series(expp.At(), &builder, &explset, &expchks)
 			require.NoError(t, err)
 			require.Equal(t, explset, lset)
 			require.Equal(t, expchks, chks)
diff --git a/tsdb/querier.go b/tsdb/querier.go
index 89e3d5719..4082279ae 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -452,12 +452,13 @@ type blockBaseSeriesSet struct {
 
 	bufChks []chunks.Meta
 	bufLbls labels.Labels
+	builder labels.ScratchBuilder
 	err     error
 }
 
 func (b *blockBaseSeriesSet) Next() bool {
 	for b.p.Next() {
-		if err := b.index.Series(b.p.At(), &b.bufLbls, &b.bufChks); err != nil {
+		if err := b.index.Series(b.p.At(), &b.builder, &b.bufLbls, &b.bufChks); err != nil {
 			// Postings may be stale. Skip if no underlying series exists.
 			if errors.Cause(err) == storage.ErrNotFound {
 				continue
diff --git a/tsdb/querier_test.go b/tsdb/querier_test.go
index e6a75da2f..9933b7158 100644
--- a/tsdb/querier_test.go
+++ b/tsdb/querier_test.go
@@ -1270,7 +1270,7 @@ func (m mockIndex) SortedPostings(p index.Postings) index.Postings {
 	return index.NewListPostings(ep)
 }
 
-func (m mockIndex) Series(ref storage.SeriesRef, lset *labels.Labels, chks *[]chunks.Meta) error {
+func (m mockIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
 	s, ok := m.series[ref]
 	if !ok {
 		return storage.ErrNotFound
@@ -1884,9 +1884,10 @@ func TestPostingsForMatchers(t *testing.T) {
 		p, err := PostingsForMatchers(ir, c.matchers...)
 		require.NoError(t, err)
 
+		var builder labels.ScratchBuilder
 		for p.Next() {
 			lbls := labels.Labels{}
-			require.NoError(t, ir.Series(p.At(), &lbls, &[]chunks.Meta{}))
+			require.NoError(t, ir.Series(p.At(), &builder, &lbls, &[]chunks.Meta{}))
 			if _, ok := exp[lbls.String()]; !ok {
 				t.Errorf("Evaluating %v, unexpected result %s", c.matchers, lbls.String())
 			} else {
@@ -2097,7 +2098,7 @@ func (m mockMatcherIndex) SortedPostings(p index.Postings) index.Postings {
 	return index.EmptyPostings()
 }
 
-func (m mockMatcherIndex) Series(ref storage.SeriesRef, lset *labels.Labels, chks *[]chunks.Meta) error {
+func (m mockMatcherIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
 	return nil
 }
 
diff --git a/tsdb/repair_test.go b/tsdb/repair_test.go
index 6d95fc0a6..d39411560 100644
--- a/tsdb/repair_test.go
+++ b/tsdb/repair_test.go
@@ -80,11 +80,12 @@ func TestRepairBadIndexVersion(t *testing.T) {
 	require.NoError(t, err)
 	p, err := r.Postings("b", "1")
 	require.NoError(t, err)
+	var builder labels.ScratchBuilder
 	for p.Next() {
 		t.Logf("next ID %d", p.At())
 
 		var lset labels.Labels
-		require.Error(t, r.Series(p.At(), &lset, nil))
+		require.Error(t, r.Series(p.At(), &builder, &lset, nil))
 	}
 	require.NoError(t, p.Err())
 	require.NoError(t, r.Close())
@@ -106,7 +107,7 @@ func TestRepairBadIndexVersion(t *testing.T) {
 
 		var lset labels.Labels
 		var chks []chunks.Meta
-		require.NoError(t, r.Series(p.At(), &lset, &chks))
+		require.NoError(t, r.Series(p.At(), &builder, &lset, &chks))
 		res = append(res, lset)
 	}