Organosulfonic acid-functionalized mesoporous silicas were tested for catalytic performance in the esterification of oleic acid with methanol in flowing supercritical carbon dioxide. The energy of activation of the sulfonic acid catalysts was found to be about 42 kJ/mol and was shown to be independent of pore size. The catalytic activity of the functionalized silicas was compared to a standard acidic resin, Amberlyst 15, and to an immobilized lipase, Novozym 435. The most active catalyst was Novozym 435. (C) 2006 Elsevier B.V. All rights reserved.