The existence of mass transport limitations in catalytic chain transfer mediated emulsion polymerization of methyl methacrylate (MMA) using bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF) was investigated. The rate of mass transport of COBF from the aqueous phase toward the polymer particles proved to depend strongly on the viscosity of the polymer particles and consequently on the instantaneous conversion. At high instantaneous conversion the exchange of COBF between the particles and the aqueous phase is severely hindered. As a result, the control of the molecular weight distribution is hampered. At low instantaneous conversion COBF is readily transferred between the aqueous phase and polymer particles, resulting in immediate molecular weight control. The chain transfer activity of COBF inside the polymer particles during the polymerization was successfully quantified using the chain length distribution method. The results of this work show that the presence of a catalytic chain transfer agent can severely affect the course of the emulsion polymerization. Furthermore, it was demonstrated that molecular weight control during the final stages of an emulsion polymerization is affected by the increasing viscosity of the polymer particles. © 2009 American Chemical Society.