Dihetaryl thioketones react with thiocarbonyl ylides to give 1,3-dithiolanes in high yields. No competitive side reactions of the thiocarbonyl ylides were observed, evidencing the 'superdipolarophilic' character of this less-known group of thioketones. Depending on the type of substituents present in both the thiocarbonyl ylide and the thioketone, formal [3+2] cycloadditions occur with complete regioselectivity or with formation of a mixture of both regioisomers. Regioselective formation of the sterically more crowded 1,3-dithiolanes is explained via a mechanism involving stabilized 1,5-biradicals. In systems with less-efficient radical stabilization, e.g., in the case of adamantanethione S-methanide, substantial violation of the regioselectivity was observed as a result of steric hindrance.