The effect of water on the alcoholysis of methyl propionate and n-propanol catalyzed by immobilized Candida antarctica lipase B (CALB) has been compared in a continuous solid/gas reactor and in an organic liquid medium. The enthalpic and entropic contributions of water to the Gibbs free energy of activation in the gas phase were different from the ones in the organic phase, the inverse trends being observed for the variation of both ∃H* and ∃S* with water activity. Different phenomena were identified for their influence on the thermodynamic parameters. When increasing aW, the enhanced flexibility of the enzyme was predominant in the gas phase whereas enzyme-solvent interactions due to an increased polarity of the solvent affected mainly the thermodynamic parameters in the organic phase. The observed variations of ∃G* with water activity were in accordance with kinetics results previously obtained in both reaction media.