Treatment of solutions of 1,3-thiazole-5(4H)-thiones 1 in CH2Cl2 at room temperature with BF3 .Et2O and 1,2-epoxycyclohexane (7-oxabicyclo[4.1.0]heptane; 2a) or 1,2-epoxycyclopentane (6-oxabicyclo[3.1.0]hexane; 2b) yielded mixtures of diastereoisomeric spirocyclic 1,3-oxathiolanes (3/4 and 8/9, respectively). In addition, the corresponding 1,3-dithiolane 6 was formed as a minor product in the reaction of 4,4-dimethyl-2-phenyl-1,3-thiazole-5(4H)-thione (1a) with 2a. The structures of the different types of products have been established by X-ray crystal-structure analysis. An ionic two-step mechanism via nucleophilic ring-opening of the complexed oxirane by the attack of the thiocarbonyl S-atom is proposed. This proposal is supported by the formation of the propellane 10 with a Wagner-Meerwein rearrangement as the key step.