The addition of monodisperse size crosslinked polystyrene (PS) particles, synthesized by emulsifier-free emulsion polymerization, to polysulfide matrix enhanced mechanical properties of the cured rubbery composites. The modulus, fracture strength, and elongation at break increased with increasing filler volume fraction up to 30 wt % PS particles. The strength and elongation at break decreased with increasing particle diameter from 0.315 to 1.25 mu m. The strength at break increased, but the extension decreased, as the particle crosslink density increased from 0 to 5 mol % DVB. Interparticle interactions are dominant and lead to the formation of clusters which form a network structure in PS particle filled composites. Since the number density, as well as the total surface area, of particles increase with decreasing particle diameter, interparticle attractions are enhanced, the tendency for cluster formation increased with decreasing particle size from 1.25 to 0.315 mu m. As particle crosslink density was reduced, the porosity and surface roughness of particles increased. Then, the dispersion of particles in the matrix was enhanced and particle agglomeration reduced but more polymer matrix was adsorbed on the particles. These particles or clusters act as physical crosslinks, resulting in an increased total effective crosslink density in the filled composites.