Six novel 3-alkyl(aryl)-4-(p-nitrobenzoylamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones (2a-f) were synthesized by the reactions of 3-alkyl(aryl)-4-amino-4,5-dihydro-1H-1,2,4- triazol-5-ones (1a-f) with p-nitrobenzoyl chloride and characterized by elemental analyses and IR, (1)H-NMR, (13)C-NMR and UV spectral data. The newly synthesized compounds 2 were titrated potentiometrically with tetrabutylammonium hydroxide in four non-aqueous solvents such as acetone, isopropyl alcohol, tert-butyl alcohol and N, N-dimethylformamide, and the half-neutralization potential values and the corresponding pKa values were determined for all cases. Thus, the effects of solvents and molecular structure upon acidity were investigated. In addition, isotropic (1)H and (13)C nuclear magnetic shielding constants of compounds 2 were obtained by the gauge-including-atomic-orbital (GIAO) method at the B3LYP density functional level. The geometry of each compound has been optimized using the 6-311G basis set. Theoretical values were compared to the experimental data. Furthermore, these new compounds and five recently reported 3-alkyl-4-(2-furoylamino)- 4,5-dihydro-1H-1,2,4-triazol-5-ones (3a-c,e,f) were screened for their antioxidant activities.