The esterification of fatty acids is an important pretreatment step in the production of biodiesel from low-cost raw materials. It was thus used as a model reaction in a study of the influence of concentration variations of the polar component (methanol) in a reaction mixture on the morphology and functioning of ion exchange catalysts. For a gel-type ion exchange catalyst, a decrease in the methanol concentration proved to have unambiguously negative effects, as it induced de-swelling of the polymer matrix and diminished the accessibility of the active centers. For macroreticular ion exchange catalysts, on the other hand, the drop in methanol concentration had a positive influence on the efficiency of the catalytic action mechanism of the acidic centers. This was due to the existence of some swelling-independent porosity, causing a shift from a general to a specific catalysis-like mechanism. (C) 2010 Elsevier B.V. All rights reserved.