Full and complete esterification of oleic acid and methanol, catalyzed by cationic activation, is done quickly at room temperature in a new kind of reactor fitted with cation-exchange membranes, REMI. This type of reactor displays liquid phases and prevents the occurrence of the compensating ion of the proton, which catalyses the reaction, in the reaction medium. Some common inorganic (hydrochloric, phosphoric, sulphuric) and organic (acetic, methane sulfonic, paratoluene sulfonic) acids were used as catalysts. With such a reactor, ester is produced quickly at room temperature without mixing with the catalysing acid. No downstream processing is required. A dimensionless kinetic law is proposed for the present experiments and is suitable for all possible reaction mechanisms when cationic activation occurs with only one of the reactants.