The invention relates to a method for reducing the quantity of residual monomers in aqueous dispersions of polymers by re-treatment with an initiator system which essentially comprises the following: a) 0.001 to 5 wt. % (in relation to the total quantity of monomers used to produce the polymer dispersion) of a1) an oxidising agent R OOH, R being hydrogen, a C1- to C8-alkyl group or a C6- to C12-aryl group and/or a2) a compound which releases hydrogen peroxide in an aqueous medium and b) 0.005 to 5 wt. % (in relation to the total quantity of monomers used to produce the polymer dispersion) of b1) an alpha -hydroxycarbonyl compound of formula (I), wherein R and R , independently of each other, mean hydrogen and/or a C1- to C12- alkyl group which can contain functional groups and/or be olefinically unsaturated and R and R optionally form a ring structure with methylene groups and said ring structure can contain functional groups and/or be olefinically unsaturated and/or b2) a compound which releases alpha -hydroxycarbonyl compounds of this type in an aqueous medium and c) advantageously, catalytic quantities of a multivalent metal ion which can occur in several valence states.