Thiol-thione tautomerism of 1,2,4-triazole derivative with Schiff base was investigated by spectro-scopic methods and quantum mechanical calculations. Theoretical study of thiol-thione tautomeric forms of 1,2,4-triazole derivative with Schiff base; 1,2,4-triazole-thiol form, 1-((5-mercapto-4-(thio-phene-2-ylmethyleneamino)-4H-1,2,4-triazole-3-yl)methyl)-3-(thiophene-2-ylmethyl)-4-(thiophene-2- ylmethyleneamino)-1H-1,2,4-triazole-5(4H)-one (I) and 1,2,4-triazole-thione form, 3-(thiophene2-ylmethyl)-4-(thiophene-2-ylmethyleneamino)-1-((4-(thiophene-2-ylmethyleneamino)-5-thioxo-4,5- dihydro-1H-1,2,4-triazole-3-yl)methyl)-1H-1,2,4-triazole-5(4H)-one (II) was performed by the density functional theory (DFT) method with 6-311++G(d,p) basis set. Structural parameters were obtained and spectral parameters of NMR, FTIR and U-V vis were compared with experimental ones to determine structural details. In vitro antileishmanial activity was studied against Leishmania infantum promastigots by microdilution broth assay with Alamar Blue Dye. The results indicate that 1,2,4-triazole derivative exists in both thiol and thione form and, can be evaluated as antiparasitic in term of antileishmanial activity. (C) 2017 Elsevier B.V. All rights reserved.