Abstract: The invention relates to the use of a mixture of(a1) a naturally occurring, zwitterionic phosphatide or a mixture of naturally occurring zwitterionic phosphatides and(a2) cholic acid and/or a salt of cholic acid and/or deoxycholic acid and/or a salt of deoxycholic acid and/or ricinoleic acid and/or a salt of ricinoleic acid and/or oleic acid and/or a salt of oleic acidas an emulsifier for the preparation of aqueous polymer dispersions which can be prepared by free radical emulsion polymerization of monomers which can be polymerized by free radicals or mixtures of monomers which can be polymerized by free radicals.