The catalytic chain transfer agent [bis[mu-[(2,3-butanedione dioximato)(2-)-O,O’]] tetrafluorodiborato(2-)-N,N’,N " N"’]cobalt (COBF) was investigated in polymerizations of methyl methacrylate and styrene in bulk and in methyl methacrylate in toluene solution. A wide range of polymer molecular weights were produced by these reactions under identical polymerization conditions, enabling the use of these reactions in the examination of the chain length dependence of the average termination rate coefficient. A strong chain length dependence in accord with theoretical predictions for diffusion-controlled termination was observed. A total of five variations of the use of the Mayo equation and the shape of the number distribution were compared as methods of evaluating the chain transfer activity of COBF. MALDI-TOF mass spectrometry and SEC were evaluated for measuring the number distributions of the polymers and deficiencies were exhibited in the distributions from MALDI-TOF. Both the number distribution and Mayo treatments of the data gave similar results. COBF was shown to have a very high chain transfer activity in all of these polymerizations. Chain transfer activities of ca. 36000 were found for COBF in bulk MMA, ca. 25000 for MMA in toluene solution, and ca. 1500 for bulk styrene. A marked solvent effect is shown for toluene in the COBF-mediated polymerization of MMA.