S-Allylcysteine (SAC) is the most abundant compound in aged garlic extracts, and its antioxidant properties have been demonstrated. It is known that SAC is able to scavenge different reactive species including hydroxyl radical ((center dot)OH), although its potential ability to scavenge peroxyl radical (ROO(center dot)) has not been explored. In this work the ability of SAC to scavenge ROO(center dot) was evaluated, as well as the role of the allyl group (-S-CH(2)-CH=CH(2)) in its free radical scavenging activity. Two derived compounds of SAC were prepared: S-benzylcysteine (SBC) and S-propylcysteine (SPC). Their abilities to scavenge (center dot)OH and ROO(center dot) were measured. A computational analysis was performed to elucidate the mechanism by which these compounds scavenge (center dot)OH and ROO(center dot). SAC was able to scavenge (center dot)OH and ROO(center dot), in a concentration-dependent way. Such activity was significantly ameliorated when the allyl group was replaced by benzyl or propyl groups. It was shown for the first time that SAC is able to scavenge ROO(center dot).