Organosulfonic acid-functionalized mesoporous silicas were synthesized in a one-step approach of co-condensing inorganic-organic reagents in the presence of different surfactant templates with in situ oxidation of the thiol groups to the sulfonic acid groups. The resulting, materials were tested for their catalytic performance in the esterification of fatty acid with methanol to produce methyl esters. The performance of the functionalized mesoporous materials demonstrated a strong dependence on the median pore diameter of the catalyst as well as the acidic strength of the organosulfonic acid group. The activity of the organosulfonic acid-functionalized silicas in the esterification was compared to that of standard acidic resins. The results indicate the potential of rational catalysis design using organic-inorganic mesoporous materials. (C) 2003 Elsevier Inc. All rights reserved.