A process for reducing the amount of residual monomer in aqueous polymer dispersions operates by aftertreatment with an initiator system essentially comprising a) from 0.001 to 5% by weight, based on the total monomer amount used to prepare the polymer dispersion, a.sub.1) of an oxidizing agent R.sup.1 OOH, where R.sup.1 is hydrogen or a C.sub.1 -C.sub.8 -alkyl or a C.sub.6 -C.sub.12 -aryl group, and/or a.sub.2) of a compound which in aqueous medium releases hydrogen peroxide, and b) from 0.005 to 5% by weight, based on the total monomer amount used to prepare the polymer dispersion, b.sub.1) of an .alpha.-hydroxycarbonyl compound ##STR1## where R.sup.2 and R.sup.3 independently of one another are hydrogen and/or a C.sub.1 -C.sub.12 -alkyl group which may contain functional groups and/or may be olefinically unsaturated, or R.sup.2 and R.sup.3 optionally, by way of methylene groups, form a ring structure which may contain functional groups and/or may be olefinically unsaturated, and/or b.sub.2) of a compound which in aqueous medium releases such .alpha.-hydroxycarbonyl compounds, and c) advantageously, catalytic amounts of a polyvalent metal ion which is able to exist in a plurality of valence states.