The novel 5-alkyl(aryl)-4-(aryl)-2,4-dihydro-3H-1,2,4-triazol-3-ones (1-12) were synthesized by reaction of (N′- ethoxycarbonyl)-4-alkyl(aryl)hydrazonic acid ethyl ester with primary amines by microwave irradiation. Stoichiometric protonation constants of 1-12 were determined potentiometrically in 50% (v/v) amfiprotic (ethanol and methanol) and dipolar aprotic (dioxan)-water mixtures at 25oC with an ionic strength of 0.10 M. The calculation of the stoichiometric protonation constants was carried out using a PKAS computer program. The effects of solvents composition on the stoichiometric protonation constants are discussed.