Abstract: Neutral sizing of base paper pulp in a conventional aqueous suspension at a neutral pH for the production of sized, acid-free base paper by internal sizing with the use of aqueous cationic copolymer dispersions having a minimum cation activity of 20 .mu.mol/g of solids, more than half of the cationic charge being located on the surface of the dispersion copolymer particles, in combination with polymeric retention aids, the weight ratio of the polymeric retention aid to the cationic dispersion copolymer preferably being 0.3:1 to 0.005:1, in a quantity of up to 2% by weight of cationic dispersion copolymer, relative to the dry weight of the raw cellulose fibers.