The present invention relates to an aqueous dispersion polymerization process for preparing a (co)polymer wherein an organic peroxide is used as initiator (as a source of free radicals) during the polymerization process in conjunction with an effective amount of an organic peroxide stabilizing additive (controlling agent). The invention also relates to formulations comprising an organic peroxide and an effective amount of an organic peroxide stabilizing additive suitable for use in said polymerization process. The invention finally relates to 10 (co)polymers obtainable by the dispersion polymerization process.