The esterification of oleic acid by ethanol using lipases from Rhizopus niveus and Mucor miehei as catalysts was evaluated in microaqueous, biphasic (n-hexane-water) and surfactant-enriched biphasic systems containing different amounts of water. The results showed that for R. niveus lipase-catalyzed esterification in a microaqueous system, a water content corresponding to a water activity of 0.75 is optimal from a kinetic point of view. In biphasic systems, the influence of the water content on the thermodynamics of the reaction and the effect of the interfacial area between phases on the velocity of the biocatalysis were demonstrated. In addition, the water content was shown to influence the velocity of the reaction in the surfactant-enriched biphasic system. It was also demonstrated that, from a kinetic point of view, both lipases are by far more efficient in biphasic systems. From a thermodynamic point of view, biphasic systems allow interesting esterification rates to be obtained whereas they were shown to be less suitable than the microaqueous system to carry on interesterification reactions.