The effects of the sweep fluid on solid-state polymerization (SSP) of poly(bisphenol A carbonate) (BPA-PC) were investigated. Prepolymers with two different number-average molecular weights, PCP6C (M-n 3800 g/mol) and PCP9C (M-n = 2400 g/mol), were synthesized using melt transesterification. Solid-state polymerization of these prepolymers was carried out at temperatures in the range of 150-190 degrees C with supercritical carbon dioxide (scCO(2)) and N-2 as the sweep fluids. It was found that scCO(2) at 207 bar could either increase or decrease the rate of SSP relative to the rate in atmospheric N-2, depending on the prepolymer molecular weight. At 190 degrees C, the molecular weights of the polymers synthesized from the higher-molecular-weight prepolymer (PCP6C) were higher with scCO(2) as the sweep fluid compared to those of the polymers synthesized with N-2. In contrast, at the same temperature of 190 degrees C, the molecular weights of the polymers synthesized from the lower-molecular-weight prepolymer (PCP9C) were lower with scCO(2) compared to those of the polymers synthesized with N-2. This apparently contradictory effect can be understood in terms of competition between rate-increasing effects of scCO(2), such as greater chain mobility and a higher byproduct diffusion coefficient caused by plasticizing of the polymer by scCO(2), and rate-diminishing effects associated with scCO(2), such as lamellar thickening, higher crystallinity, and perhaps increased occlusion of end groups in crystalline regions.