A polymerization reactor for producing aqueous polymer dispersions by radically initiated emulsion polymerization of one or more ethylenically unsaturated monomers in the presence of one or more protective colloids and/or emulsifiers, is cleaned by initially charging an aqueous mixture comprising one or more ethylenically unsaturated monomers, one or more protective colloids and/or emulsifiers, but no oxidation initiator, the proportion of ethylenically unsaturated monomers being 20 to 70% by weight of the aqueous mixture, heating the aqueous mixture to a temperature of 50° C. to 100° C. for 5 minutes to 3 hours, adding one or more initiators and optionally further starting materials, initiating polymerization, removing the aqueous polymer dispersion thus obtained from the polymerization reactor and post-treating the dispersion to remove wall deposits detached during the process.