The use of equations such as D-pore = D-Bulk (epsilon/tau) to predict pore diffusivities in the modeling of liquid phase polymerizations on heterogeneous Ziegler-type catalysts leads to highly unrealistic results when applied to situations with activities greater than 10,000 g/g/h or higher (polyethylene in suspension). A simple, isothermal model of mass transfer with reaction is presented and is used to examine the slurry polymerization of ethylene at activities higher than those previously studied in order to explore the major tendencies in the development of concentration gradients and average molecular weight of the polymer and to evaluate accepted estimates of monomer diffusivity in the catalyst pores. Experimental results are compared with the predictions of the classic reaction diffusion model, and it is shown that values of monomer diffusivity commonly used to model slurry polymerisations are not high enough in order to correctly simulate the activity levels obtained in this work. The modeling study shows that the effect of mass transfer resistance on the molecular weight is not all together negligible and that either estimates of the diffusion coefficient of ethylene in the catalyst pores need to be revised, or that more complete description of mass transfer is required than is provided by the classic reaction/diffusion equations.