Aqueous alkyl ketene dimer dispersions which comprise at least 15% by weight of an alkyl ketene dimer and at least one water-soluble cationic starch, aluminum sulfate and at least one condensate of naphthalenesulfonic acid and formaldehyde or lignin sulfonic acid or in each case salts thereof, the dispersions comprising, based in each case on alkyl ketene dimer, from 2 to 50% by weight of aluminum sulfate, and in which the dispersions additionally comprise from 0.1 to 5% by weight of a saturated carboxylic acid having 1 to 10 carbon atoms, benzenesulfonic acid, p-toluenesulfonic acid and/or a mineral acid at an aluminum sulfate content of from 2 to 15% by weight, and the use of the dispersions as engine size and surface size for paper and paper products.