PURPOSE: To obtain the title compound, useful as an intermediate for the synthesis of drugs or the like, and having a germicidal activity, by reacting an easily available ketone compound, e.g. desoxybenzoin with nitrogen tetrasulfide.

CONSTITUTION: A ketone compound, e.g. p-chlorobenzyl-phenyl ketone, of formula I (Ar is phenyl or pyridyl group which may be substituted; R is alkyl, phenalkyl, or phenyl group which may be substituted) is reacted with nitrogen tetrasufide, preferably in a solvent, e.g. toluene, to give a 3,4-disubstituted-1,2,5-thiadiazole, e.g. the compound of formula III, of formula II. The nitrogen tetraulfide is stable unless a high temperature or very severe shock is applied, particularly in a solvent.