Abstract: A process for producing styrene-containing polymers having a narrow particle size distribution, comprising the steps:(i) preparing a polymerization mixture, said mixture comprisinga) an organic phase comprising at least 50 wt %. styrene and a monomer-soluble polymerization initiator, andb) an aqueous phase comprising water, an organic protective colloid and a substantially water insoluble inorganic suspension stabilizer;(ii) adding to said polymerization mixture 50-500 ppm of a metal carbonate, bicarbonate or mixtures thereof based on said aqueous phase; and(iii) polymerizing said polymer mixture to produce said styrene-containing polymer particles.