Full and complete esterification of oleic acid and methanol, catalyzed by cationic activation, is performed quickly at room temperature in a new kind of reactor, REMI, fitted with cation-exchange membranes. Use of ion-exchange membranes involves liquid phases, prevents the appearance of the compensating ion of the proton which catalyses the reaction, in the reaction medium and allows pertraction of water from the reacting mixture. Three kind of cation-exchange membranes were tested.