Mesoporous MCM-41 and -48 silicas anchored with sulfonic acid (-SO3H) groups via postsynthesis modification are very effective for the synthesis of Bisphenol-A by liquid-phase condensation of phenol with acetone. Higher amounts of thiol groups can be incorporated in MCM-48 silicas presumably due to the presence of larger number of surface silanol groups. However sulfur K-edge XANES spectroscopy reveals that effective oxidation of the precursor thiol (-SH) groups to the sulfonic acid (-SO3H) groups was necessary for obtaining samples of good catalytic activity. It was noted that when sulfur loadings exceed 1.5 meq/g solid a part of the sulfur atoms remains in the reduced form even after prolonged oxidation. MCM-41 silica anchored with sulfonic acid groups has comparable catalytic activity to that of commercial ion-exchange resin Amberlite-120 and the former also showed higher selectivity toward the desired p, p' isomer. MCM-48 silica-anchored samples are equally effective for selective synthesis of Bisphenol-A. (C) 2004 Elsevier Inc. All rights reserved.