In heterogenous olefin polymerization with Ziegler catalysts, the influence of monomer mass transport in the growing granule on polymer properties has been extensively modeled, but it has not been possible to clearly establish the importance of diffusion experimentally since the multisited nature of most Ziegler catalysts can produce similar effects. In this study, ethylene-propylene copolymers were made with single-sited metallocene catalysts by slurry polymerization in liquid monomers. These copolymers had a relatively narrow molecular weight distribution with a composition distribution (CD) broader than expected for a single-site catalyst. Data analysis indicates that mass-transfer limitations in the polymer particles are the most likely explanation for the observed results. For amorphous copolymers, a diffusion/reaction model could predict CD breadth in good agreement with experimental data, but for semicrystalline polymers the model was in accurate. We postulate that model inadequacies are due to radial gradients in monomer diffusivity during polymerization which the model does not account for.