A method for reducing the amount of residual monomer in aqueous polymer dispersions by aftertreatment with an initiator system comprises aftertreating the aqueous polymer dispersion with addition of 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, of an inorganic salt of persulfuric acid, b) from 0.005 to 5% by weight, based on the total monomer amount used to prepare the polymer dispersion, of a methyl ketone, and c) optionally, catalytic amounts of a metal ion which is able to exist in a plurality of valence states.