For some polymers, the rate of solid-state polymerization (SSP) is higher with supercritical carbon dioxide (scCO(2)) as the sweep gas than with atmospheric N-2. One explanation for this higher rate is that the diffusion coefficient of the condensate molecule is higher in the CO2-swollen polymer. To investigate this hypothesis, we measured the diffusion coefficient of phenol in poly(bisphenol A carbonate) (BPA-PC) by carrying out SSP of this polymer under diffusion-limited conditions. Under these conditions, the diffusion coefficient of the condensate molecule could be calculated from the profile of the molecular weight versus time. The phenol diffusivity was determined between 135 and 180 degreesC in the presence of N-2 at about 1 bar and in the presence of SCCO2 at about 138, 207, and 345 bar. The diffusion coefficient of phenol was up to 200% higher in ScCO2 than in N-2, depending on the temperature and CO2 pressure. With both N-2 and ScCO2, the activation energy for phenol diffusion in BPA-PC was larger than the activation energy for the reaction between hydroxyl and phenyl end groups that occurred during SSP of BPA-PC. As a result, the overall SSP reaction shifted from diffusion control at low temperatures toward chemical-reaction control at high temperatures. (C) 2003 Wiley Periodicals, Inc.