Cyclohydrocarbonylative ring expansion of acetylenic thiazoles in the presence of CO, H-2, and catalytic quantities of the zwitterionic rhodium complex (eta (6)-C6H5BPh3)Rh--(+)(1,5-COD) and triphenyl phosphite affords thiazepinones in 61 to 90% yields. This novel transformation of a 5- to a 7-membered heterocycle is readily applied to acetylenic thiazoles containing hydro, alkyl, alkyl halide, vinyl, and benzo substitutents in positions 4 and 5 of the thiazole ring in addition to alkyl-, ether-, ester-, vinyl-, and aryl-substituted alkynes at position 2.