Two mesomeric heterocyclic betaines, phenyl pyrimidinium betaine (BT1) and undecyl pyrimidinium betaine (BT2), were investigated for their antioxidant activity using the 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. The antioxidant propriety was assessed by the effective concentration of the betaine (EC50), the time required to reach the steady state of DPPH discoloration at EC50 concentration (TEC50), the antiradical efficiency (AE), and the reduction kinetics . Vitamin E (a-tocopherol) and di-tert-butylhydroxytoluene (BHT) were used as the standard antioxidants in in vitro assays. Kinetic studies showed that BT1 was more effective than vitamin E and BT2.