The precipitation polymerization of acrylic acid (AA) has been carried out at 50 and 70 degrees C in supercritical carbon dioxide (ScCO2), using a continuous stirred-tank reactor. Three kinetic models are compared with experimental data on the rate of polymerization and the viscosity-average molecular weight. The first model, the "solution polymerization" model, is based on the assumption that all reactions: initiation, propagation, and termination, take place in the fluid phase; no reaction takes place in the polymer phase. In the second model, the "surface polymerization" model, chain initiation is assumed to occur in the fluid phase, whereas chain propagation and termination occur in a thin zone on the surface of the polymer particles. The third model, the "interior polymerization" model, is similar to the "surface polymerization" model, except that chain propagation and chain termination take place uniformly throughout each polymer particle. Monomer is assumed to be in phase equilibrium between the supercritical fluid and the polymerization zone. The surface polymerization and interior polymerization models both provide a much better description of the experimental data than the solution polymerization model. This suggests that a significant portion of AA polymerization takes place in the polymer phase, when scCO(2) is the reaction medium. However, the data do not support a choice between the surface and interior polymerization models. (c) 2005 Elsevier Ltd. All rights reserved.