HPLC-MS analysis of 4-amino-5-(4-nitrophenyl)-2,4-dihydro-3H-1,2,4-triazole-3-thione tautomeric derivatives has been performed and composition of the tautomers mixture has been set. The relationship between the structure of synthesized compounds and their UV-spectrums has been studied, the results of atomic charges calculation have been described. Proofs conformity peaks in the chromatogram thione and thiol. Based on these data, we consider that the major component is thione and the minor one is thiol.