An in vitro antioxidant assay has been developed to better reflect the in vivo conditions of antioxidants interacting with membrane and lipid surfaces. The lipid peroxidation inhibition capacity (LPIC) method measures the ability of both lipophilic and hydrophilic antioxidants to protect a lipophilic fluorescent probe 4, 4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid, incorporated in the membrane, from 2,2′-azobis(2-amidinopropane)hydrochloride generated radicals in the surrounding aqueous solution. Antioxidant activities of test compounds were measured either after they were mixed with preformed liposomes (LPICMixed) or after they were incorporated into liposomes (LPICInco) as they were made. The results were analysed to determine how the method of mixing and the structures of the antioxidants influenced their protection of the membrane from free radical attack. The LPICMixed values were larger than the LPICInco values for a range of 12 structurally diverse antioxidant compounds. However, there was no linear correlation between the lipophilicities, as measured by their partition coefficient, log P and either LPICInco or LPICMixed values. A strong correlation was found between LPICInco and LPICMixed values.