The electron structures of 3-substituted 1,2,4-triazines and isomeric N-methyltriazinium salts were calculated by the CNDO/2 method. The results of the calculations were compared with the parameters of the 13C, 15N, and 14N NMR spectra, as well as with experimental data on the protonation, N-alkylation, and reactions of 1,2,4-triazines with simple nucleophiles. The protonation centers of 1,2,4-triazines were determined by means of 1H, 13C, and 14N NMR spectroscopy. The covalent adducts formed by the proton salts of 3-methoxy-, 3-methylthio-, 3-morpholino-, and 3-pyrrolidino-1,2,4-triazine with water and methanol were recorded by 1H NMR spectroscopy.