The treatment of 2-hydrazinopyrimidines (1) with acylating agents under appropriate conditions is used to prepare s-triazolo[4,3-a]pyrimidine (6a) and its derivatives bearing alkyl or aryl groups at the 3-, 5-, 6- , or 7-position. These compounds rearrange in acid or alkali to give the corresponding s-triazolo[1,5-a]pyrimidines (7). Rates for such isomerizations in alkali increase steeply in the range pH 10-12.5; in acid, they reach maxima at pH values corresponding approximately to the pKa of each substance (in the range pH 1.5-2.5) and fall off on both sides. Rearrangement of the parent heterocycle (6a) is slowed significantly by each added 3-, 6-, and 7-alkyl group and slowed profoundly by a 5-alkyl group: for example, 3,5,7-trimethyltriazolo[4,3-a]pyrimidine rearranges 2500 times more slowly than the unsubstituted heterocycle at pH 11.3. The isomeric systems (6) and (7) are distinguished easily by their tabulated u.v. and N.M.R. spectra.