The effects of particle polydispersity on the normalized overall rate constant of spherical dispersions under the diffusion-controlled situation are investigated. Two industrially important but inherently different particle size distributions, normal and log-normal distributions, are studied. A numerical scheme for generating normally or log-normally distributed particles given a fixed particle volume fraction acid normalized standard deviation or geometric standard deviation is developed to facilitate the study. Results for the two particle size distributions show dramatically different behavior. For dispersions with normally distributed particles, the normalized overall rate constant is almost invariant with respect to variation in the normalized standard deviation, while that for dispersions containing log-normally distributed particles decreases with increasing particle size spread. It is further found that, for the normal distribution case, the overall rate constant fits well with a universal scaling relation, proposed by Torquato and Yeong based on curve-fitting of rate constants of 8 classes of dispersion microstructure, and a specific surface area based scaling relation. The overall rate constant of the log - normal case however cannot be described by either scaling relation. The normalized overall rate constants of both distributions increase, as they should, with increasing particle volume fraction.