The ring expansion reactions of 1,3-oxathiolium salts (1) using NaN3 and an I2–aq NH3 system resulted exclusively in the formation of 1,4,2-oxathiazines (3) by incorporation of a nitrogen atom into the C–O bond in 1. On the other hand, the reaction of 1 with dialkylamino(thioxo)methanesulfenamides, another type of N-expansion reagent, gave di-5-thiazolyl sulfide derivatives (4) in addition to 3. The