The catalytic chain transfer polymerization of CO2-expanded butyl methacrylate and styrene is reported. Experimentally determined values of the chain transfer constant for CO2-expanded butyl methacrylate are presented at 50 and 60 degrees C and in the range of pressure from 0.1 to 6 MPa, using a cobaloxime complex as the chain transfer catalyst. Similar data are reported for CO2-expanded styrene at 50 degrees C. The chain transfer constants for both expanded monomers are significantly higher than those obtained in the bulk monomers. It is demonstrated that a linear relationship exists between the chain transfer rate coefficient and the inverse of viscosity. These results provide significant evidence that the rate-determining step in the chain transfer process is diffusion-controlled.