Several free radical addition polymerizations exhibit the diffusion limited gel or Trommsdorff effect. The models available until recently could not be applied to industrially important semibatch operations and to nonisothermal polymerizations. The recent model of Ray et al. (24) can indeed be applied to such situations. In this study, an experimental setup to study polymerizations under such conditions has been assembled. Methyl methacrylate (MMA) is polymerized in a 1-liter stainless steel, computer-controlled reactor. A series of experiments on bulk polymerization of MMA under different temperature histories (near-isothermal, step decrease, and step increase) at a fixed initiator (AIBN) concentration of 25.8 mol/m(3) have been conducted. Two important process output variables, namely, monomer conversion and average molecular weight, have been measured off-line at different sampling times during the course of polymerization. The conversion and molecular weight data are found to be in reasonable accord with model (24) predictions obtained using the experimental (nonisothermal) temperature histories, and best fit correlations for three parameters evaluated using earlier data from the literature on isothermal batch reactors (with no further curve fitting). The new model (24) can, thus, be used with some degree of confidence to design and control larger scale reactors.