The effect of diffusion on linear, step-growth polymerizations has been quantified via "chain-by-chain" simulations of the polymerization process and a diffusion model consistent with reptation theories. The results show that diffusion limitations can narrow the molecular weight distribution and alter the way it evolves during polymerization; e.g., when diffusion is the limiting step, the polydispersity index (PDI1 = M-w/M-n) converges to a value of similar to1.3 at complete conversion. Nevertheless, diffusion limitations have little effect on the time dependence of conversion. Consequently (for this type of polymerization), modeling of diffusion phenomena is important only when an accurate calculation of the MWD is needed. We also show that the numerical value of the Hatta number, at the point of entanglement (phi), can be used to determine whether diffusion limitations are likely to be important in a given system. Guidelines for the estimation of p are given.