diff --git a/tests/test_postprocessing.py b/tests/test_postprocessing.py
index 3ceabcb4..18dc6455 100644
--- a/tests/test_postprocessing.py
+++ b/tests/test_postprocessing.py
@@ -7,8 +7,8 @@
 import xesmf
 
 from xmip.postprocessing import (
+    _construct_and_promote_member_id,
     _parse_metric,
-    _promote_member_id,
     combine_datasets,
     concat_experiments,
     concat_members,
@@ -249,7 +249,7 @@ def _assert_parsed_ds_dict(ddict_parsed, expected, match_keys, strict=True):
     _assert_parsed_ds_dict(ds_dict_parsed, ds_metric[metricname], ["a"])
 
 
-def test_match_metrics_missing_attr():
+def test_match_metrics_missing_non_match_attr():
     """This test ensures that as long as the provided `match_metrics` are
     given they will be matched. This is relevant if e.g. the variant label
     has been removed due to merging"""
@@ -559,8 +559,11 @@ def test_concat_members(concat_kwargs):
     # Group together the expected 'matches'
     # promote the member_id like in concat_members
     expected = {
-        "a.a.a.a": [_promote_member_id(ds_a_temp), _promote_member_id(ds_b_temp)],
-        "c.a.a.a": [_promote_member_id(ds_c_other)],
+        "a.a.a.a": [
+            _construct_and_promote_member_id(ds_a_temp),
+            _construct_and_promote_member_id(ds_b_temp),
+        ],
+        "c.a.a.a": [_construct_and_promote_member_id(ds_c_other)],
     }
 
     result = concat_members(
@@ -579,6 +582,109 @@ def test_concat_members(concat_kwargs):
         assert member in result["a.a.a.a"].member_id
 
 
+def test_concat_members_existing_member_dim():
+    attrs_a = {
+        "source_id": "a",
+        "grid_label": "a",
+        "experiment_id": "a",
+        "table_id": "a",
+        "variant_label": "a",
+        "version": "a",
+    }
+
+    attrs_b = {k: v for k, v in attrs_a.items()}
+    attrs_b["variant_label"] = "b"
+
+    # Create some datasets with a/b attrs
+    ds_a = random_ds(attrs=attrs_a).rename({"data": "temp"})
+    ds_b = random_ds(attrs=attrs_b).rename({"data": "temp"})
+
+    ds_a_promoted = ds_a.expand_dims({"member_id": [ds_a.attrs["variant_label"]]})
+    ds_b_promoted = ds_b.expand_dims({"member_id": [ds_b.attrs["variant_label"]]})
+
+    # testing mixed case
+    ds_dict = {"some": ds_a_promoted, "thing": ds_b}
+
+    # promote the member_id like in concat_members
+    expected = xr.concat([ds_a_promoted, ds_b_promoted], "member_id")
+
+    result = concat_members(
+        ds_dict,
+    )
+
+    xr.testing.assert_equal(
+        result["a.a.a.a"],
+        expected,
+    )
+
+
+def test_concat_members_existing_member_dim_different_warning():
+    attrs_a = {
+        "source_id": "a",
+        "grid_label": "a",
+        "experiment_id": "a",
+        "table_id": "a",
+        "variant_label": "a",
+        "version": "a",
+    }
+
+    attrs_b = {k: v for k, v in attrs_a.items()}
+    attrs_b["variant_label"] = "b"
+
+    # Create some datasets with a/b attrs
+    ds_a = random_ds(attrs=attrs_a).rename({"data": "temp"})
+    ds_b = random_ds(attrs=attrs_b).rename({"data": "temp"})
+
+    ds_a_promoted_wrong = ds_a.expand_dims({"member_id": ["something"]})
+
+    # testing mixed case
+    ds_dict = {"some": ds_a_promoted_wrong, "thing": ds_b}
+    msg = "but this is different from the reconstructed value"
+    # TODO: Had trouble here when putting in the actual values I expected.
+    # Probably some regex shit. This should be enough for now
+    with pytest.warns(UserWarning, match=msg):
+        concat_members(
+            ds_dict,
+        )
+
+
+def test_concat_members_reconstruct_from_sub_experiment_id():
+    attrs_a = {
+        "source_id": "a",
+        "grid_label": "a",
+        "experiment_id": "a",
+        "table_id": "a",
+        "variant_label": "a",
+        "version": "a",
+    }
+
+    attrs_b = {k: v for k, v in attrs_a.items()}
+    attrs_b["variant_label"] = "b"
+    attrs_b["sub_experiment_id"] = "sub_something"
+
+    # Create some datasets with a/b attrs
+    ds_a = random_ds(attrs=attrs_a).rename({"data": "temp"})
+    ds_b = random_ds(attrs=attrs_b).rename({"data": "temp"})
+
+    ds_a_promoted = ds_a.expand_dims({"member_id": ["a"]})
+    ds_b_promoted = ds_b.expand_dims({"member_id": ["sub_something-b"]})
+
+    # testing mixed case
+    ds_dict = {"some": ds_a, "thing": ds_b}
+
+    # promote the member_id like in concat_members
+    expected = xr.concat([ds_a_promoted, ds_b_promoted], "member_id")
+
+    result = concat_members(
+        ds_dict,
+    )
+
+    xr.testing.assert_equal(
+        result["a.a.a.a"],
+        expected,
+    )
+
+
 @pytest.mark.parametrize("concat_kwargs", [{}, {"compat": "override"}])
 def test_concat_experiments(concat_kwargs):
     concat_kwargs = {}
@@ -905,7 +1011,7 @@ def test_nested_operations():
     )
     ddict = {"ds1": ds_1, "ds2": ds_2, "ds3": ds_3, "ds4": ds_4}
 
-    ddict = {k: _promote_member_id(ds) for k, ds in ddict.items()}
+    ddict = {k: _construct_and_promote_member_id(ds) for k, ds in ddict.items()}
 
     ds_expected = xr.Dataset(
         {
diff --git a/xmip/postprocessing.py b/xmip/postprocessing.py
index 238b9886..43145b2a 100644
--- a/xmip/postprocessing.py
+++ b/xmip/postprocessing.py
@@ -197,8 +197,26 @@ def merge_variables(
     )
 
 
-def _promote_member_id(ds):
-    ds = ds.assign_coords(member_id=ds.attrs["variant_label"])
+def _construct_and_promote_member_id(ds):
+    # construct member_id according to https://docs.google.com/document/d/1h0r8RZr_f3-8egBMMh7aqLwy3snpD6_MrDz1q8n5XUk/edit
+    sub_experiment_id = ds.attrs.get("sub_experiment_id", "none")
+    variant_label = ds.attrs[
+        "variant_label"
+    ]  # if this should fail in the future, we could build an error/warning in here
+    if sub_experiment_id == "none":
+        member_id = variant_label
+    else:
+        member_id = f"{sub_experiment_id}-{variant_label}"
+
+    if "member_id" not in ds.dims:
+        ds = ds.expand_dims({"member_id": [member_id]})
+    else:
+        existing_member_id = ds.member_id.data[0]
+        if not existing_member_id == member_id:
+            warnings.warn(
+                f"{cmip6_dataset_id(ds)} already contained a member_id ({existing_member_id}) but this is different from the reconstructed value ({member_id}). The existing value is not modified, but we recommend checking the input.",
+                UserWarning,
+            )
     return ds
 
 
@@ -234,7 +252,7 @@ def concat_members(
 
     # promote variant_label attr to coordinate, to have member_ids as coordinates
 
-    ds_dict = {k: _promote_member_id(ds) for k, ds in ds_dict.items()}
+    ds_dict = {k: _construct_and_promote_member_id(ds) for k, ds in ds_dict.items()}
 
     return combine_datasets(
         ds_dict,