Bisphenol A is an important starting material for production of epoxy resins and polycarbonates, which is manufactured by ion exchanger resin catalyzed condensation of acetone and phenol. Much higher reaction rate is achieved when in the reaction environment are present meracpto-group containing compounds. Alternatively, the promoter can be either dissolved in the reaction mixture or immobilized in the resin catalyst. Important problem of the process using the promoted catalyst is relatively rapid catalyst deactivation. Analysis of the reactor operation and mathematical modeling of its behavior indicate that the cause of the deactivation is a poison in the reaction mixture inhibiting the promoter. Proper design of laboratory experiments allows investigation of the deactivation process within acceptable time period and testing of possible preventive actions. (C) 2004 Elsevier B.V. All rights reserved.