High value-added specialty composite materials can now be made by in-situ suspension polymerization of monomers with additives incorporated to impart a specific property. In these composite particles, the nature of the additive dispersion is often of critical importance in determining final product properties. During polymerization, viscosities in the droplets are sufficiently low that significant diffusion can occur, resulting in dramatic changes to the additive dispersion quality. This article examines the importance of process parameters controlling diffusion lengths of small particle additives during in-situ polymerizations. A model was developed to provide a basis for devising polymer reactor operating strategies that minimize small-particle diffusion. Effects of varying polymerization rate, molecular weight and initial conversion were investigated. Model results were validated by comparing to experimental data. Increasing the initial conversion is shown to be highly effective in reducing diffusion of additive particles.