Theoretical analysis has been performed to determine the effect of internal diffusion on a second-order reversible esterification of acetic acid with methanol catalyzed by a solid acid catalyst, Amberlyst 15. The results of the analysis show that the catalyst effectiveness factors are above 0.94 for the beads smaller than 0.6 mm diameter at the reaction temperature lower than 80 degrees C. However, the effectiveness factors were lower than 0.77 for the beads larger than 1.0 mm diameter at reaction temperatures higher than 94 degrees C. For the commercial Amberlyst 15 with an average particle size of 0.7 mm diameter, an overall effectiveness factor of 0.87 was predicted for a reaction temperature of 94 degrees C. The predicted effectiveness factors agree with those obtained from the kinetic measurements.