The present invention relates to alkoxy-groups-containing copolymers of a) 1 to 99%-wt. water soluble ethylenically unsaturated, acid-groups-containing monomers, b) 1 to 50%-wt. monomers having a polyalkoxy sequence and c) 0 to 60%-wt. water soluble monomers which are copolymerizable with a) and b); and to the use of said copolymers for retanning leathers previously tanned by a mineral or combined method. The leathers treated with the retanning agents according to the invention in particular stand out for an improved fullness and good grain tightness.The acid groups of monomers a) preferably are carboxyl groups.