The mechanism of catalytic chain transfer polymerization with methyl methacrylate was studied in a range of media. The chain transfer reaction in supercritical CO2 was found to be significantly enhanced compared with similar experiments in toluene or in bulk methyl methacrylate. The results are consistent with a diffusion-controlled rate-determining step in the transfer process with cobaloxime catalysts. The gaslike viscosities in the supercritical medium result in an approximate chain transfer coefficient (k(tr)) of 10(8) L mol(-1) s(-1): an order of magnitude higher than values obtained in conventional organic solvents.