Astaxanthin (AST) is a naturally occurring pinkish carotenoid of marine organisms, such as alga, crustacean (crab, shrimp, and lobster) and fish (salmon and trout). Several studies have demonstrated the remarkable antiperoxidative activity of AST when associated with biological membranes or biomimetic models. AST was shownto scavenge oxyradicals both at the surface (in the AST terminal ring moiety) and in the interior of phospholipid membranes (at the AST conjugated polyene chain), which could, at least in part, explain its high antioxidant capacity. We study here the efficiency of AST as an inhibitor of lipoperoxidation induced by equimolar concentrations of peroxynitrite and cytochrome c, a cationic heme protein that strongly attaches to biological membranes. The oxidizing system was tested at different pH conditions (6.2; 6.8; 7.4; 8.0) against AST-loaded egg-yolk phosphatidylcholine unilamellar liposomes (PCL^AST) and levels of produced thiobarbituric acid-reactive substances (TBARS) were compared to control (PCL, without AST) to estimate the protective role of AST in the zwitterionic vesicles. In order to increase electrostatic interactions between cytochrome c and the lipid bilayers, anionic liposomes were also prepared by incorporating 5% and 10% egg yolk phosphatidylglycerol in the lipid composition. Maximum scavenging efficiency of AST in all lipid preparations was observed at pH 7.4, with dramatic decreases towards acidic conditions. Interestingly, AST did not exhibit significant antioxidant capacity in anionic liposomes at pH 6.2 or 6.8, where AST instead behaves as a prooxidant. These data show that the antioxidant capacity of AST is not governed only by the carotenoid policonjugated structure but is strikingly dependent on the radical chemistry, membrane composition and reaction conditions.