The effect of bulk viscosity on the cobaloxime-mediated catalytic chain-transfer polymerization of methacrylates at 60 degreesC was investigated by both the addition of high molecular weight poly(methyl methacrylate) to methyl methacrylate polymerization and the dilution of benzyl methacrylate polymerization by toluene. The results indicate that the bulk viscosity is not directly linked to the chain-transfer activity. The previously measured relationship between chain-transfer-rate coefficient and monomer viscosity therefore probably reflects changes at the molecular level. However, the results in this article do not necessarily disprove a diffusion-controlled reaction rate because cobaloxime diffusion is expected to scale with the monomer friction coefficient rather than bulk viscosity. Considering the published data, to date we are not able to distinguish between a diffusion-controlled reaction rate or a mechanism directly affected by the methacrylate substituent.