Carbon-13 nmr spectra of 18 thiazoles with different substituents (R = CH3, C6H5, Cl, Br, NH2et N3) have been recorded. The13C chemical shifts and thenJ(C,H) coupling constants are discussed as a function of the nature of the substituent and the electronic structure of the thiazole ring. The 2-azido substituted thiazoles show azido-tetrazole isomerism, making possible a comparative study of substituent effects in thiazole and thiazolotetrazole rings. These studies have been extended to other heterocycles: benzothiazole, isothiazole, and isoxazole.