The free-radical propagation rate coefficients for methyl methacrylate (MMA) in liquid and supercritical carbon dioxide have been successfully determined using pulsed-laser polymerization (PLP) method. The effect of carbon dioxide solvency on the propagation rate coefficient, k(p), is examined at 180 bar for a temperature range of 20-80 degrees C. The temperature dependency of the obtained k(p) data is evaluated by use of the Arrhenius equation. The resulting activation energy is (25.4 +/- 1.2 kJ.mol(-1), and the preexponential factor is (5.2 +/- 3.0 x 10(6) L.mol(-1).s(-1). These results are compared to the bulk activation parameters for the propagation rate coefficient of MMA as set forth by the IUPAC working party on "Modeling of Kinetics and Processes of Polymerization". The discrepancy in the propagation rate coefficients at the lower temperature regime is attributed to a solvent effect of CO2. This is consistent with the results from a recent PLP study of this monomer.