<p>A process for preparing expandable styrene polymer particles by using magnesium oxide as a suspending agent to prevent agglomeration of styrene polymer particles during impregnation with an expanding agent. The process comprises preparing expandable styrene polymer particles from styrene polymer particles in water, wherein the expanding agent (which does not dissolve styrene polymer particles or slightly swells the particles, and has a boiling point lower than the softening point of the particles) is added in the presence of the suspending agent and the particles being impregnated thereby.</p>
