In the presence of Keggin-type phosphovanadomolybdic acids, e.g., H₆PV₃Mo₉O₄₀, oxidative dimerization of various kinds of primary thioamides including aromatic, heterocyclic, and aliphatic ones efficiently proceeded to give the corresponding 3,5-disubstituted 1,2,4-thiadiazoles in excellent yields.