In this work the effects of conversion on the apparent catalyst activity in the catalytic chain transfer polymerization of methyl methacrylate are reported. Several mechanisms are discussed that may explain the experimental observations. The discussion is supported with computer simulations using Predici software. It is shown that the experimental decrease in weight average molecular weight with conversion is smaller than the decrease obtained in simulations. The most likely cause for this discrepancy is slow catalyst deactivation. The half-life of CoBF under the reported conditions was determined to be about 10 h. Furthermore, the effect of acetic acid (HAc) and benzoyl peroxide (BPO) on the evolution of the molecular weight distribution is investigated. Both HAc and BPO enhance catalyst deactivation. For HAc, catalyst deactivation scales with the square root of its concentration. BPO- enhanced deactivation depends linearly on its concentration. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1375-1388, 2004.