The influence of water on the kinetics of alcoholysis of methyl propionate and n-propanol catalyzed by immobilized lipase B from Candida antarctica was studied in a continuous solid/gas reactor. In this reactor, the solid phase is composed of a packed enzymatic sample which is percolated by gaseous nitrogen, simultaneously carrying gaseous substrates to the enzyme while removing reaction products. In this system, interactions between the enzyme and non-reacting molecules are avoided, since no solvent is present, and it is thus more easy to assess the role of water. To this end, alcohol inhibition constant, substrates dissociation constants as well as acylation rate constant and ratio of acylation to deacylation rate constants have been determined as a function of water activity (aW). Data obtained highlight that n-propanol inhibition constant and dissociation constant of methyl propionate are a lot affected by aW variations whereas water has no significant effect on the catalytic acylation step nor on the ratio of acylation to deacylation rate constants. These results suggest the water independent character of the transition step.