Intraparticle mass transfer can play an important role in catalyzed polymerizations. This work discusses the importance of taking this influence into account or not when modeling a fluidized bed reactor. An analysis was made of this influence on the reactor phenomena, on polymer productivity and on the polymer's physicochemical properties (polymer quality). Two basic cases were simulated: low and high residence time of the polymer particle inside the reactor (low: 10 min; high: 1 h). The results indicate the need to model the reactor taking into account the intraparticle mass transfer and the concentration profile inside the particle when the fluidized bed operates under conditions of low residence time. For high residence times (above 20 min), the overall simulation results change only slightly when intraparticle mass transfer is not considered. (C) 2002 Elsevier Science Ltd. All rights reserved.266841848