A process for the preparation of cationic paper sizing agents is disclosed. A copolymer of maleic anhydride and an .alpha.-olefin containing 10 to 26 carbon atoms is first reacted with from 0.1 to 1.0 mol, per mol of anhydride, of a monoprimary/monotertiary alkylene diamine, subsequently reacting the copolymer with more than 0.2 and less than 1.5 mols, per mol of anhydride, of a low-molecular weight polyamine, with the proviso that the total amount of amine reacted be less than 2.0 mols per mol of anhydride, and converting the resultant product to a salt.