The role of single crystal diffraction in the quantitative determination of polymorphism is demonstrated by the examination of three compounds. Two polymorphs were found for each of the compounds bis(2-nitrophenyl) trisulphide (1), 2-amino-5-nitrobenzophenone (2) and bis(2-nitrophenyl) sulphide (3). Only in one polymorph of (1) does molecular symmetry correspond with crystallographic symmetry. In (2) the polymorphs arise in the same crystal class and in the same crystallographic space group whereas in (3) the two polymorphs exist in different crystal classes and hence in different space groups. Crystallographic space group transformation is also discussed.