AbstractA series of fused 5,5‐diphenyl and 5‐arylidene‐2‐thiohydantoin derivatives were examined for their oxygen free radical inhibitory and radical scavenging properties (RSC) using both an enzymic and non‐enzymic biological generators of free radicals. Non‐enzymic lipid peroxidation (OH radicals) was assayed as the amount of malondialdehyde (MDA) that had been formed during incubation of boiled rat liver microsomes in the presence of ascorbic acid and ferric ions. Superoxide anions (02− anion radicals) were generated enzymatically in the xanthine‐xanthine oxidase system. Among the 20 investigated compounds only four fused arylidene 2‐thiohydantoin derivatives showed weak antioxidant activity scavenging OH radicals. There is a relationship between the electrophilic OH radical scavenging properties (RSC) of the tested molecules and the value of their HOMO energy derived from semiempirical MO calculations.