The invention is directed to the use of maleic esters selected from compounds of formulae (I) and (II) as copolymerizable emulsifiers in the emulsion polymerization of olefinically unsaturated monomers, in which A is an alkyl group having 3 to 40 carbon atoms; X and Z independently are hydrogen or methyl; M is hydrogen, an alkali metal, alkaline earth metal, or an ammonium or other amine salt; and n and p independently are numbers in the range from 0 to 40.