Carboxyl-containing polymers which are water-soluble at least in the neutralized form are prepared by a method in which a monoethylenically unsaturated C.sub.3 -C.sub.5 -carboxylic acid is polymerized in the presence or absence of a carboxyl-free copolymerizable ethylenically unsaturated monomer in aqueous solution in the presence of polymerization initiator and of from 0.01 to 5% by weight, based on the monomers, of an inorganic acid of phosphorus in which the phosphorus has an oxidation state of from 1 to 4, its water-soluble salts, a water-soluble --PO(OH).sub.2 -containing compound and/or its water-soluble salts and, if required, a polymerization regulator. Virtually colorless or only slightly colored polymer solutions are obtained.