The molecular and crystal structures and tautomerism of protonated isomeric 3-pyridyl- substituted 5-amino-1H-1,2,4-triazoles were studied within the framework of density function- al theory and by X-ray analysis. According to B3LYP/6-311++G(2d,2p) calculations of six possible tautomeric forms A-F, tautomers A with the N-protonated pyridine moiety are predominant in the gas phase. 5-Amino-4H-1-onium compounds (tautomers D) of higher polarity are next in stability; their relative energies decrease as follows: 3-pyridin-4-yl-, 3-pyridin-3-yl-, and 3-pyridin-2-yl-5-amino-4H-1,2,4-triazol-1-onium cation. A comparison of experimental 13C NMR spectra of tautomers A and D with their NMR spectra calculated by the GIAO method showed that both tautomeric forms can coexist in comparable amounts in DMSO solutions. Tautomers A and D were detected by X-ray analysis in crystals of 5-amino-3-(pyrid- in-2-yl)-1H-1,2,4-triazolium salts.