Aqueous polymer dispersions comprising effect substances with a mean particle size of the dispersed particles of <1000 nm, in which the polymer particles comprise a polymer matrix of at least one ethylenically unsaturated monomer, at least 0.1% by weight of a polymer from the group consisting of (i) at least one homopolymer of ethylene, propylene, 1-butene, 2-butene, 1-pentene or 1-hexene with an average molar mass M.sub.W from 100 to 10 000, (ii) at least one copolymer of at least two of the monomers mentioned under (i) with an average molar mass from 100 to 1000 and/or (iii) polyisobutylene with an average molar mass M.sub.W of at least 100and at least one effect substance which is soluble in the ethylenically unsaturated monomers from which the polymers are formed, processes for the preparation of such dispersions by miniemulsion polymerization, in which a solution of at least one effect substance in at least one ethylenically unsaturated monomer and at least one polymer according to (i), (ii) and/or (iii) is emulsified in water in the presence of at least one surface-active agent to give a miniemulsion with a mean droplet size <500 nm and the miniemulsion is subsequently polymerized under radical conditions, and also use of the aqueous polymer dispersions thus obtainable or the polymer powders obtained therefrom for the stabilizing of polymers against the action of UV radiation, oxygen and/or heat, as lubricants for polymers, in cosmetic or pharmaceutical formulations, in surface coatings, in the preparation of paper, leather or textiles, in formulations for feeding animals and formulations for agriculture and forestry.