diff --git a/setup.py b/setup.py
index 9d37014..fb77d5e 100644
--- a/setup.py
+++ b/setup.py
@@ -8,7 +8,7 @@
 
 setup(
     name="pyqn",
-    version="1.4",
+    version="1.4.1",
     description="A package for managing physical units and quantities",
     long_description=long_description,
     long_description_content_type="text/x-rst",
diff --git a/src/pyqn/dimensions.py b/src/pyqn/dimensions.py
index ecaf6fb..b41d6a6 100644
--- a/src/pyqn/dimensions.py
+++ b/src/pyqn/dimensions.py
@@ -47,8 +47,7 @@ def __init__(self, dims=None, **kwargs):
             if not kwargs:
                 self.dims = dims
             else:
-                print("bad initialisation of Dimensions object")
-                sys.exit(1)
+                raise ValueError("Bad initialisation of Dimensions object")
         else:
             # initialize by keyword arguments
             for dim_name in kwargs:
diff --git a/src/pyqn/units.py b/src/pyqn/units.py
index ac6e6c7..9b2ea6c 100644
--- a/src/pyqn/units.py
+++ b/src/pyqn/units.py
@@ -29,6 +29,10 @@
 h, NA, c, kB = (6.62607015e-34, 6.02214076e23, 299792458.0, 1.380649e-23)
 
 
+class UnitsConversionError(UnitsError):
+    pass
+
+
 class Units:
     """
     A class to represent the units of a physical quantity.
@@ -327,25 +331,25 @@ def kBT_conversion(self, other):
         else:
             raise UnitsError(
                 "Failure in conversion of units: was expecting to "
-                "covert between energy and temperature"
+                "convert between energy and temperature"
             )
         return fac / other.to_si()
 
     def mol_conversion(self, other):
-        from_dims = self.get_dims()  # original unit dimensions
-        to_dims = other.get_dims()  # desired unit dimensions
-        fac = self.to_si()  # factor needed to conver to SI units
+        from_dims_Q = self.get_dims().dims[4]
+        to_dims_Q = other.get_dims().dims[4]
 
-        if from_dims.dims[4] == to_dims.dims[4]:
-            raise UnitsError(
-                "Failure in conversion of units: no "
-                "different in quantity dimensions between %s and %s" % from_dims,
-                to_dims,
+        # We can only remove or add the amount dimension in its entirity.
+        if from_dims_Q and to_dims_Q:
+            raise UnitsConversionError(
+                f"Cannot force molar conversion between {self} and {other}"
             )
-        elif from_dims.dims[4] > to_dims.dims[4]:
-            fac = fac / (NA ** (from_dims.dims[4] - to_dims.dims[4]))
+
+        fac = self.to_si()
+        if from_dims_Q:
+            fac = fac * NA**from_dims_Q
         else:
-            fac = fac * (NA ** (to_dims.dims[4] - from_dims.dims[4]))
+            fac = fac * NA**to_dims_Q
         return fac / other.to_si()
 
     def spec_conversion(self, other):
diff --git a/tests/test_conversions.py b/tests/test_conversions.py
index c31dda1..abc0842 100644
--- a/tests/test_conversions.py
+++ b/tests/test_conversions.py
@@ -31,13 +31,51 @@ def test_litres(self):
         self.assertEqual(u1.to_si(), 1.0e-3)
 
     def test_molar_units_conversion(self):
+
+        u1 = Units("mol")
+        u2 = Units("1")
+        self.assertAlmostEqual(u1.conversion(u2, force="mol"), 6.02214076e23)
+
+        u1 = Units("mol-1")
+        u2 = Units("1")
+        self.assertAlmostEqual(u1.conversion(u2, force="mol"), 1 / 6.02214076e23)
+        print(u1.dims.dims)
+
+        u1 = Units("mol2")
+        u2 = Units("1")
+        self.assertAlmostEqual(u1.conversion(u2, force="mol"), 6.02214076e23**2)
+
+        u1 = Units("mol-2")
+        u2 = Units("1")
+        self.assertAlmostEqual(u1.conversion(u2, force="mol"), 6.02214076e23**-2)
+
         u1 = Units("kJ")
-        u2 = Units("J/mol")
+        u2 = Units("kJ.mol-1")
 
         with self.assertRaises(UnitsError) as cm:
             u1.conversion(u2, strict=True)
         self.assertAlmostEqual(u1.conversion(u2, force="mol"), 1.660e-21)
 
+        u3 = Units("cal.mol-1")
+        u4 = Units("eV")
+        self.assertAlmostEqual(u3.conversion(u4, force="mol"), 4.3364104350063916e-05)
+
+        u9 = Units("nmol2.L-1")
+        u10 = Units("L-1")
+        self.assertAlmostEqual(u9.conversion(u10, force="mol"), 3.626617933325338e29)
+
+        u5 = Units("J.mmol-1")
+        u6 = Units("J")
+        self.assertAlmostEqual(u5.conversion(u6, force="mol"), 1.6605390671738467e-21)
+
+        u7 = Units("1")
+        u8 = Units("mol-2")
+        self.assertAlmostEqual(u7.conversion(u8, force="mol"), 2.757389993610589e-48)
+
+        u9 = Units("nmol2.L-1")
+        u10 = Units("L-1")
+        self.assertAlmostEqual(u9.conversion(u10, force="mol"), 3.626617933325338e29)
+
     def test_kBT_units_conversion(self):
         u1 = Units("K")
         u2 = Units("J")