Conditions have been established for the direct reaction of thiosemicarbazides with carboxylic acids in the presence of polyphosphate ester (PPE) to synthesize 1,2,4-triazole-3-thiol derivatives. The synthesis involves two consecutive steps: (i) acylation of the thiosemicarbazide with a carboxylic acid in chloroform in the presence of PPE at 90 °C using a hydrothermal reaction vessel, followed by (ii) cyclodehydration of the acylation product by treatment with an aqueous alkali solution. Using this new synthetic approach, 15 derivatives of 1,2,4-triazole-3-thiol were obtained, five of which were synthesized for the first time. The structures of the synthesized compounds were confirmed by NMR spectroscopy and mass spectrometry.