diff --git a/orsopy/fileio/base.py b/orsopy/fileio/base.py
index 87b2c855..cf8154b8 100644
--- a/orsopy/fileio/base.py
+++ b/orsopy/fileio/base.py
@@ -870,7 +870,7 @@ def _read_header_data(file: Union[TextIO, str], validate: bool = False) -> Tuple
                 # numerical array  and start collecting the numbers for this
                 # dataset
                 _d = np.array([np.fromstring(v, dtype=float, sep=" ") for v in _ds_lines])
-                data.append(_d)
+                data.append(_d.T)
                 _ds_lines = []
 
                 # append '---' to signify the start of a new yaml document
@@ -883,7 +883,7 @@ def _read_header_data(file: Union[TextIO, str], validate: bool = False) -> Tuple
 
         # append the last numerical array
         _d = np.array([np.fromstring(v, dtype=float, sep=" ") for v in _ds_lines])
-        data.append(_d)
+        data.append(_d.T)
 
         yml = "".join(header)
 
diff --git a/orsopy/fileio/orso.py b/orsopy/fileio/orso.py
index 2f830015..be551caa 100644
--- a/orsopy/fileio/orso.py
+++ b/orsopy/fileio/orso.py
@@ -166,8 +166,11 @@ class OrsoDataset:
     data: Union[np.ndarray, Sequence[np.ndarray], Sequence[Sequence]]
 
     def __post_init__(self):
-        if self.data.shape[1] != len(self.info.columns):
+        if len(self.data) != len(self.info.columns):
             raise ValueError("Data has to have the same number of columns as header")
+        column_lengths = set(len(c) for c in self.data)
+        if len(column_lengths) > 1:
+            raise ValueError("Columns must all have the same length in first dimension")
 
     def header(self) -> str:
         """
@@ -252,13 +255,13 @@ def save_orso(
 
         ds1 = datasets[0]
         header += ds1.header()
-        np.savetxt(f, ds1.data, header=header, fmt="%-22.16e")
+        np.savetxt(f, np.asarray(ds1.data).T, header=header, fmt="%-22.16e")
 
         for dsi in datasets[1:]:
             # write an optional spacer string between dataset e.g. \n
             f.write(data_separator)
             hi = ds1.diff_header(dsi)
-            np.savetxt(f, dsi.data, header=hi, fmt="%-22.16e")
+            np.savetxt(f, np.asarray(dsi.data).T, header=hi, fmt="%-22.16e")
 
 
 def load_orso(fname: Union[TextIO, str]) -> List[OrsoDataset]:
diff --git a/orsopy/fileio/tests/test_orso.py b/orsopy/fileio/tests/test_orso.py
index 5ff3823a..8fc32772 100644
--- a/orsopy/fileio/tests/test_orso.py
+++ b/orsopy/fileio/tests/test_orso.py
@@ -107,8 +107,8 @@ def test_write_read(self):
         # test write and read of multiple datasets
         info = fileio.Orso.empty()
         info2 = fileio.Orso.empty()
-        data = np.zeros((100, 3))
-        data[:] = np.arange(100.0)[:, None]
+        data = np.zeros((3, 100))
+        data[:] = np.arange(100.0)[None, :]
 
         info.columns = [
             fileio.Column("Qz", "1/angstrom"),
@@ -190,14 +190,14 @@ def test_unique_dataset(self):
         info2.data_set = 0
         info2.columns = [Column("stuff")] * 4
 
-        ds = OrsoDataset(info, np.empty((2, 4)))
-        ds2 = OrsoDataset(info2, np.empty((2, 4)))
+        ds = OrsoDataset(info, np.empty((4, 2)))
+        ds2 = OrsoDataset(info2, np.empty((4, 2)))
 
         with pytest.raises(ValueError):
             fileio.save_orso([ds, ds2], "test_data_set.ort")
 
         with pytest.raises(ValueError):
-            OrsoDataset(info, np.empty((2, 5)))
+            OrsoDataset(info, np.empty((5, 2)))
 
     def test_user_data(self):
         # test write and read of userdata
@@ -208,8 +208,8 @@ def test_user_data(self):
             fileio.ErrorColumn("R"),
         ]
 
-        data = np.zeros((100, 3))
-        data[:] = np.arange(100.0)[:, None]
+        data = np.zeros((3, 100))
+        data[:] = np.arange(100.0)[None, :]
         dct = {"ci": "1", "foo": ["bar", 1, 2, 3.5]}
         info.user_data = dct
         ds = fileio.OrsoDataset(info, data)
@@ -247,7 +247,7 @@ def test_save_numpy_scalar_dtypes(self):
         info = fileio.Orso.empty()
         info.data_source.measurement.instrument_settings.wavelength = Value(np.float64(10.0))
         info.data_source.measurement.instrument_settings.incident_angle = Value(np.int32(2))
-        ds = fileio.orso.OrsoDataset(info, np.arange(20.).reshape(10, 2))
+        ds = fileio.orso.OrsoDataset(info, np.arange(20.).reshape(2, 10))
         # .ort test:
         fileio.save_orso([ds], "test_numpy.ort")
         ls = fileio.load_orso("test_numpy.ort")
diff --git a/orsopy/fileio/tests/test_schema.py b/orsopy/fileio/tests/test_schema.py
index 10cfb2fd..aa55698d 100644
--- a/orsopy/fileio/tests/test_schema.py
+++ b/orsopy/fileio/tests/test_schema.py
@@ -18,7 +18,7 @@ def test_example_ort(self):
             schema = json.load(f)
 
         dct_list, data, version = _read_header_data(pth / "test_example.ort", validate=True)
-        assert data[0].shape == (2, 4)
+        assert data[0].shape == (4, 2)
         assert version == "0.1"
 
         # d contains datetime.datetime objects, which would fail the
@@ -34,4 +34,4 @@ def test_example_ort(self):
         assert len(dct_list) == 2
         assert dct_list[1]["data_set"] == "spin_down"
         assert data[1].shape == (4, 4)
-        np.testing.assert_allclose(data[1][2:], data[0])
+        np.testing.assert_allclose(data[1][:, 2:], data[0])