The main purpose of the study was the development of a new method for synthesis of 1,3,4-thiadiazol-2-amine derivatives in a one-pot manner using the reaction between a thiosemicarbazide and carboxylic acid without toxic additives such as POCl₃ or SOCl₂. The reaction was investigated in the presence of polyphosphate ester (PPE). It was found that, in the presence of PPE, the reaction between the thiosemicarbazide and carboxylic acid proceeds in one-pot through three steps with the formation of corresponding 2-amino-1,3,4-thiadiazole. The experimental conditions for the reaction between benzoic acid and thiosemicarbazide in the presence of PPE with predominant formation of intermediate for the thiadiazole, 2-benzyl hydrazine-1-carbothioamide were found. Using the developed approach five, 2-amino-1,3,4-thiadiazoles were synthesized. The structures of all compounds were proven by mass spectrometry, IR, and NMR spectroscopies. The new approach has potential for extended application: (1) PPE can be utilized as a mild additive for thiosemicarbazide acylation reaction by carboxylic acids; and (2) the thiosemicarbazide acylation products used for synthesis of thiadiazoles can be also utilized for triazoles synthesis.