diff --git a/.github/workflows/pytest.yml b/.github/workflows/pytest.yml
index 53b96de..6d215e9 100644
--- a/.github/workflows/pytest.yml
+++ b/.github/workflows/pytest.yml
@@ -18,6 +18,14 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         py: ['3.8', '3.9', '3.10', '3.11']
+        pandas: [ '1.*' ]
+        include:
+          - os: ubuntu-latest
+            py: '3.10'
+            pandas: '1.3'
+          - os: ubuntu-latest
+            py: '3.10'
+            pandas: '1.4'
     steps:
       - uses: actions/checkout@v3
         with:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index aa6d6d1..6e58176 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,9 +1,15 @@
 # pytcs
 
-## unreleased
+## v0.1.2
+
+### added
 
 - add support for reading `StringIO` and `BytesIO` #10
 
+### fixed
+
+- explicitly sort index in `ScopeFile.as_pandas` #16
+
 ## v0.1.1
 
 initial release with basic functions
diff --git a/pytcs/pytcs.py b/pytcs/pytcs.py
index 9e01628..fa287cd 100644
--- a/pytcs/pytcs.py
+++ b/pytcs/pytcs.py
@@ -306,7 +306,7 @@ def as_pandas(
 
         _list = [v for v in _dict.values() if not v.empty]
 
-        df = pd.DataFrame().join(_list, how="outer")
+        df = pd.DataFrame().join(_list, how="outer", sort=True)
 
         if time_fmt == "timedelta":
             df.index = pd.TimedeltaIndex((df.index * 1e6).astype(np.int64))
diff --git a/tests/test_scope_file.py b/tests/test_scope_file.py
index 403e78b..d725bba 100644
--- a/tests/test_scope_file.py
+++ b/tests/test_scope_file.py
@@ -64,7 +64,9 @@ def test_scope_file(filenames, backend, native_dtypes, use_buffer):
         )
         assert all([type(v) == np.ndarray for v in sf._data.values()])
 
-        str(sf)
+        # monotonic time
+        for c in sf:
+            assert np.allclose(np.diff(sf[c].time), sf[c].sample_time)
 
     @staticmethod
     @pytest.mark.parametrize("backend", ["pandas"])