The free-radical dispersion polymerization of methyl methacrylate was studied using the reaction calorimetry technique. An especially developed calorimeter for use with supercritical fluids was employed and a set of experiments was explicitly designed to isolate the effect of the pressure. It was found that, under marginal dispersion stability conditions, small pressure changes can have significant effects on the reaction evolution. The pressure was also found to affect the partitioning of the monomer between the two reaction phases. The heat released during the nucleation stage of the reaction was measured for the first time and permitted the discussion on the polymerization locus during this stage. (c) 2007 American Institute of Chemical Engineers.