1,3-Dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides 3A-D – sterically hindered at least at one terminus – with thiobenzophenone produce both regioisomeric 1,3-dithiolanes 4 and 5. According to quantum-chemical calculations (preceding paper), a concerted cycloaddition furnishing 2,4-substituted dithiolanes 4 competes with the formation of an intermediate C,C-biradical 9 which cyclizes