Three thiirene 1-oxides 1, which possess bulky alkyl substituents at the 2- and 3-positions, afforded alpha-iminothioketones in 59-69% yields on successive treatment with trifluoroacetic anhydride and p-toluenesulfonamide, whereas 1 furnished oxazoles in 65-89% yields on the same treatment with trifluoroacetic anhydride and p-toluamide. A mechanism involving ring-expansion of the initial intermediates, sulfilimines, is proposed for these reactions.