We previously demonstrated that coencapsulation of the potent antioxidant astaxanthin (Asx) and tocotrienols into liposomes results in synergistically higher antioxidative activity than the calculated additive activity of each individual antioxidant-containing liposome, due to intermolecular interactions between terminal ring moieties of the two antioxidants and the polyene chain and the triene moiety. We reported that intermolecular interactions depend on the stereochemistry of Asx, and change the electronic state of the Asx polyene moiety. Based on these findings, we hypothesised that antioxidants that interact with Asx at the terminal ring and polyene moieties may enhance the antioxidative activity. Herein, we selected two candidate antioxidants, capsaicin (Cap) and resveratrol, based on their structures, in which the compounds exhibit similar characteristics to tocotrienols. We evaluated the antioxidative capacities of liposomes coencapsulating Asx and the selected candidates. Based on hydroxyl radical scavenging activity, Cap was found to synergistically enhance the antioxidative activity of Asx at an optimal Asx/Cap ratio. Intermolecular interactions between Asx and Cap are necessary for the synergistic effect, and the Asx stereoisomer 3R,3'R-form (Asx-R) was predicted to most potently interact. Liposomes coencapsulating Asx-R and Cap exhibited clear synergistic antioxidative activity at an optimal ratio, whereas liposomes coencapsulating the other Asx stereoisomer and Cap did not demonstrate such activity. Computational chemistry analysis showed that changes in the electronic state of the polyene moiety of Asx-R are crucial for the synergistic activity. These results suggest that antioxidants that can change the electronic state of Asx via intermolecular interactions may enhance the function of Asx.