The free-radical dispersion polymn. of Me methacrylate was studied using the reaction calorimetry technique. An esp. developed calorimeter for use with supercrit. fluids was employed and a set of expts. 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 polymn. locus during this stage.