The development of concentration gradients in the seeded emulsion homopolymerization of styrene has been studied using a mathematical model. Under ideal conditions, emulsion polymer particles are expected to be relatively uniform across their radius. When this is true, kinetic models can be reliably used to predict the reaction rates, molecular weight, and composition (for copolymerizations). However for large particles, large radial gradients in the polymer radical concentration can develop during polymerization. While gradients in the monomer concentration are not observed, the radical concentration gradients result in non-uniform reaction rates across the particle radius, rendering conventional models unreliable. This study examines the effect of seed particle diameter and various kinetic parameters on the development of particle morphology.