Five novel 3-alkyl-4-phenylacetylamino-4,5-dihydro-1H-1,2,4-triazol-5-ones (2) were synthesized by the reactions of 3-alkyl-4-amino-4,5-dihydro-1H-1,2,4-triazol-5ones (1) with phenylacetyl chloride and characterized by elemental analyses and IR, (HNMR)-H-1, C-13-NMR and UV spectral data. The newly synthesized compounds 2 were titrated potentiometrically with tetrabutylammonium hydroxide in four non-aqueous solvents such as isopropyl alcohol, tert-butyl alcohol, acetonitrile and N,N-dimethylformamide, and the half-neutralization potential values and the corresponding pK(a) values were determined for all cases. In addition, these new compounds and five recently reported 3-alkyl-4-(p-methoxybenzoylamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones (3) were screened for their antioxidant activities.