The influence of cobalt catalytic chain transfer agents to control molecular weight in the miniemulsion polymerization of methyl methacrylate has been studied. The solubility of the cobalt catalyst was found to have a large influence on the mechanism of the reaction. Two catalysts were selected for study: cobaloxime boron fluoride (COBF), which partitions approximately equally between the water and oil phases, and tetraphenylcobaloxime boron fluoride (COPhBF) which resides exclusively in the oil phase. The COBF-mediated reaction was found to be extremely sensitive to the selection of initiator, displaying poor catalytic activity in the presence of oxygen-centered radicals (derived from potassium persulfate). In contrast, the COPhBF catalytic activity proved to be independent of initiator type, as the catalyst does not come in direct contact with the initiator-derived radicals. The results are consistent with a mechanism whereby the cobalt catalyst is poisoned or deactivated in the presence of oxygen-centered radicals. In miniemulsion polymerization the catalyst can effectively be isolated from the initiator radicals, thereby allowing a batch reaction to proceed without significant loss of catalytic activity. The conclusions are supported by comparison of chain transfer constants obtained in miniemulsion and bulk polymerizations.