A new method was proposed for the preparation of diphenylphosphinylformic acid hydrazide based on the reaction of phosphine oxide and trimethylchlorosilane in the presence of a tertiary amine, followed by interaction with chloroformic acid ester and then hydrazine hydrate. The hydrazide moiety was modified into a thiosemicarbazide moiety by reacting diphenylphosphinylformic acid hydrazide with organic isothiocyanates. It was found that in an aqueous alkaline medium, thiosemicarbazides do not heterocyclize to 1,2,4-triazole-3-thiones, as expected. As a result of the rupture of the P-C bond, diphenylphosphinic acid and a 5-thioxo-1,2,4-triazolidin-3-one were formed.