The acid dissociation constants of some 4- and/or 6-substituted 2-aminobenzothiazole and 4-substituted thiazole derivatives were determined spectroscopically. With the exception of a few, the first protonation was found to occur on the amino group. In some molecules where prototropic tautomerism is possible, a change-over in the protonation mechanism was observed. The first protonation under these circumstances was found to occur on the imino nitrogen atom. The second protonation takes place on the thiazole ring nitrogen atom.