The invention relates to an aqueous organic peroxide emulsion comprising an anti-freeze agent, a protective colloid, and a plasticiser wherein the plasticizer is an ester having a ratio of the number of carbon atoms present in the pasticiser molecule (excluding aromatic carbon atoms) to the number of ester groups in the molecule of higher than 8. These emulsions show a reduced droplet growth as compared to emulsions comprising a conventional plasticizer, i.e. the emulsions of the invention are more stable upon storage than the emulsions of the prior art. The invention emulsions are particularly useful in polymerization processes such as the polymerization of vinyl chloride monomer.