The addition of thiol to the sulfonic acid-catalyzed condensation of phenol and acetone to give bisphenol A is investigated using homogeneous and SBA-15-immobilized sulfonic acids. Inclusion of thiols into the reaction media of either type of sulfonic acid catalyst accelerates the rate of reaction and shifts the regioselectivity to favor the formation of the p,p'-bisphenol isomer (bisphenol A) over the unwanted o,p'-bisphenol isomer. By immobilizing the thiol on the same solid as the sulfonic acid. reaction rates and regioselectivities are enhanced even further from those obtained from using mixtures of these two functional groups (when they are both homogeneous. or a combination of homogeneous and heterogeneous). A mechanism is proposed to explain the reactivity and involves cooperative interactions via the proximity of the immobilized sulfonic acid and the immobilized thiol. This enhanced reactivity can only be achieved by immobilizing the two functional groups on a solid. (c) 2006 Elsevier Inc. All rights reserved.