Eight 3-alkyl (aryl)-4-(3, 4-dihydroxybenzylidenamino)-4, 5-dihydro-1H-1, 2, 4-triazol-5-ones 2 having 4, 5-dihydro-1H-1, 2, 4-triazol-5-one ring are synthesized by the reactions of 3-alkyl (aryl)-4-amino-4, 5-dihydro-1H-1, 2, 4-triazol-5-ones 1 with 3, 4-dihydroxy-benzaldehyde and their antioxidant activities are investigated. In addition, compounds 2b, 2f and 2h are titrated potentiometrically with tetrabutylammonium hydroxide in non-aqueous solvents and the half-neutralization potential values and the corresponding pKa values are determined. The structures of new compounds are established from spectral data.