The invention relates to a process for preparing an aqueous polymer dispersion comprising an emulsion polymer a polymer composed of at least 5% by weight of an ethylenically unsaturated monocarboxylic acid, dicarboxylic acid or dicarboxylic anhydride (acid polymer SP for short), and an alkoxysilane of the formula SiR1R2R3R4 in which the groups R1 to R4 independently of one another are each an organic radical with the proviso that at least one of the groups is an alkoxy group, which comprises preparing said emulsion polymer by emulsion polymerization in the presence of at least some of the alkoxysilane.