Paper sizing agents in the form of an aqueous preparation of maleic acid anhydride copolymers of which the anhydride groups had been reacted either with organic or inorganic bases or with primary-tertiary diamines and the reaction products subsequently reacted with epichlorohydrin or with inorganic or organic acids to form salts, the copolymers being made up of polymerised units of(a) from 5 to 75%, by weight, of an aromatic vinyl compound;(b) from 10 to 75%, by weight, of acrylic and/or methacrylic acid C.sub.1 -C.sub.18 alkyl esters;(c) from 5 to 35%, by weight, of maleic acid anhydride;(d) from 0 to 25%, by weight, of other vinyl monomers; and(e) from 0.1 to 10%, by weight, based on the sum of monomers (a) to (d) (which amounts to 100%, by weight), of an unsaturated aldehyde or of an enol ether derived therefrom.The copolymers are produced by radical polymerisation preferably in the absence of solvents.