In the present study 3-alkyl(aryl)-4-amino-4,5-dihydro-1H-1,2,4-triazol-5-ones 2a-i reacted with 3-methyl-thiophene-2-carboxyaldehyde to afford the corresponding 3-alkyl(aryl)-4-(3-methyl- 2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones 3a-i. The acetylation reactions of compounds 3a, 3b, 3d, 3g and 3i were investigated and compounds 4a, 4b, 4d, 4g and 4i were thus obtained. The structures of fourteen new compounds are established from the spectral data. In addition, the synthesized 3 and 4 type compounds were analyzed for their in vitro potential antioxidant activities in three different methods including reducing power, free radical scavenging and metal chelating activity. Compound 3a and 3d showed the best activity for the iron binding. On the other hand, compounds 3a-i were titrated potentiometrically with tetrabutylammonium hydroxide (TBAH) in four nonaqueous solvents (isopropyl alcohol, t-butyl alcohol, acetonitrile and N,N-dimethylformamide) because of the weak acidic properties of 4,5-dihydro-1H-1,2,4-triazol-5-one ring N-H group. Also half-neutralization potential (HNP) values and the corresponding pK(a) values were determined in all cases.