The introduction of a mesoparticle scale can be important in the description of mass-transfer effects in the heterogeneous polymerization of olefins. Recent studies have shown that a morphological size level on the order of some tenths of the size of the whole particle exists. The use of traditional models that include mass-transfer effects only at the macro- and microscales gives conflicting results when the mass-transfer resistance at the meso level becomes significant. A new model to account for these effects is shown here. (C) 2003 Wiley Periodicals, Inc.