Lipases are enzymes used in numerous reactions of industrial interest. Depending on their aqueous microenvironment, lipases can catalyze hydrolysis or, conversely, organic synthesis like interesterification. This reaction can be used as a method to modify the physical and chemical properties of fats and oils, a basic process for production of ''structured lipids''. For such synthesis reactions, thermodynamic water activity (aw) of the catalyst is generally the most important parameter to control. Actually, it will directly determine the performance of the synthesis, namely its yield, selectivity and stability. Effect of the aw on the activity of immobilized Thermomyces lanuginosus and Candida antarctica B lipases in interesterification reactions was studied. Water sorption and desorption isotherms were determined, showing a phenomenon of hysteresis for the Thermomyces lanuginosus lipase. Evaluation of the influence of aw on reaction yields revealed that the IE activity tends to increase with the water activity of immobilized Thermomyces lanuginosus lipase. In contrast, aw had little influence in the case of the Candida antarctica B lipase.