The presence of the diffusion-limited gel or Trommsdorff effect in free-radical polymerizations poses a challenge for the modeling of these reactors. The available models cannot be applied to industrial reactors because of their inability to account for nonisothermal effects and semibatch operations. Recent models have overcome these limitations. These models have already been validated for step changes in temperature. The validity of these models under semibatch reactor conditions has been established in the present investigation. Experiments have been carried out at constant temperatures (50 and 70 degrees C) and a step change in the initiator concentration from about 15.48 to 100 mol/m(3) has been effected during the course of polymerization. Experimental results on monomer conversions and average molecular weights have been found to be in reasonable agreement with model predictions. The present study establishes the applicability of these models for more semibatch reactor operations, as well as the possibility of model-based optimal control of industrial reactors.