The esterification of aqueous lactic acids solution with methanol and its reverse reaction catalyzed by acidic cation exchange resins in the H+ form were carried out in a batch system. The inhibiting effects of water and methanol on the resins were evaluated. The experimental data were correlated by a kinetic model that the inhibition by methanol and water was included. The reaction rate constants and the adsorption coefficients were determined from the experiments. The activity of acidic resin was compared with that of sulfuric acid as catalyst. (C) 1996 John Wiley & Sons, Inc.