Equimolar quantities of aroyl isothiocyanates and aminoguanidine hydrochloride react additively in aqueous methanol affording excellent yields of 1-amidino-4-aroyl(thiosemicarbazides), isolable as hydrochlorides, picrates, or free bases. 1-Amidino-4-benzoyl(thiosemicarbazide) hydrochloride is cyclised by hydrochloric or orthophosphoric acid to 2-amino-5-benzamido-1,3,4-thiadiazole, by acetic anhydride to the corresponding acetyl derivative, and by alkali to 3-mercapto-5-phenyl-1,2,4-triazole. 1-Amino-2-phenylguanidine salts undergo an analogous series of reactions. The mechanism of these addition–cyclisations is discussed and correlated with those of comparable reactions.