Many antioxidants can interact to produce synergistic interactions that can more effectively inhibit lipid oxidation in foods. Esterification of rosmarinic acid produces a variety of compounds with different antioxidant activity due to differences in polarity and thus differences in partitioning in oil, water, and interfacial regions of oil-in-water emulsions (O/W). Therefore, rosmarinic acid and rosmarinate esters provide an interesting tool to study the ability of antioxidant to interact in O/W emulsions. In O/W emulsions, rosmarinic acid (R0) exhibited the strongest synergistic interaction with ?-tocopherol while butyl (R4) and dodecyl (R12) rosmarinate esters exhibited small synergistic interaction and eicosyl rosmarinate esters (R20) exhibited slightly antagonistic interaction. Fluorescence quenching and electron paramagnetic resonance (EPR) studies showed that watersoluble rosmarinic acid (R0) exhibited more interactions with ?-tocopherol than any of the tested esters (R4, R12, R20). This was also confirmed in O/W emulsions where R0 altered the formation of ?-tocopherol quinone and ?-tocopherol increased the formation of caffeic acid from R0. This formation of caffeic acid was proposed to be responsible for the synergistic activity of R0 and ?-tocopherol since the formation of an additional antioxidant could further increase the oxidative stability of the emulsion.