According to a novel process, it is possible to prepare triazolinethione derivatives of the formulain which R1 and R2 are each as defined in the description bya) reacting hydrazine derivatives of the formula with carbonyl compounds of the formulain which R3 and R4 are each as defined in the description with potassium thiocyanate, sodium thiocyanate or ammonium thiocyanate,if appropriate in the presence of a diluent and if appropriate in the presence of an acid, andb) reacting the resulting triazolidinethione derivatives of the formula with formic acid, if appropriate in the presence of a catalyst and if appropriate in the presence of a diluent.The triazolidinethione derivatives of the formula (V) are novel.