Various reactions of 3-methyl- (Im) and 3-phenyl-3H-1, 2, 3-triazolo [4, 5-d] pyrimidine (Ip) were examined. Im was prepared by cyclization of 5-amino-4-methylaminopyrimidine (II) with nitrous acid, and Ip was prepared by the catalytic reduction of 7-chloro-3-phenyl-3H-1, 2, 3-triazolo [4, 5-d] pyrimidine (IIIp) using palladium on magnesium oxide carrier as a catalyst. In warm alkaline solution Im underwent ring fission to give 5-amino-1-methyl-1H-1, 2, 3-triazole-4-carboxaldehyde (IVm). In dilute sulfuric acid both Im and Ip underwent ring fission to yield IVm and 5-amino-1-phenyl-1H-1, 2, 3-triazole-4-carboxaldehyde (IVp), respectively, in good yields. However, the reaction of Im with the methoxide ion resulted in the formation of 7, 7'-bis [3-methyl-3H-1, 2, 3-triazolo [4, 5-d] pyrimidine] (Vm) and 3-methyl-7-(5-amino-1-methyl-1H-1, 2, 3-triazol-4-yl)-3H-1, 2, 3-triazolo [4, 5-d] pyrimidine (VIm), although the yields were poor. Both Im and Ip dimerized in the presence of the cyanide ion to give Vm and 7, 7'-bis [3-phenyl-3H-1, 2, 3-triazolo [4, 5-d] pyrimidine] (Vp), respectively, in good yields. N, N-Dimethylaniline (IX) reacted with Im in dulute sulfuric acid to give 5-amino-4-[bis [4-N, N-dimethylaminophenyl] methyl]-1-methyl-1H-1, 2, 3-triazole (VIIIm) in poor yield. VIIIm was also obtained by condensation of IX and IVm in dilute sulfuric acid.