Abstract: In the process of tanning for the production of leather comprising subjecting pickled uncured hides to the action of an aqueous liquor containing(1) chemical tanning or pretanning agents, and(2) auxiliary chemicals to tanning and recovering leather, the improvement consisting essentially of employing a water-insoluble aluminosilicate, containing bound water, of theformula(Cat.sub.2/n O).sub.x.Al.sub.2 O.sub.3.(SiO.sub.2).sub.ywherein Cat represents a cation selected from the group consisting of alkali metals, bivalent metal ions, trivalent metal ions and mixtures thereof, n represents an integer from 1 to 3 of the valence of the cation, x is an integer from 0.5 to 1.8 and y is an integer from 0.8 to 50, said aluminosilicates having an average particle size in the range of 0.1.mu. to 5 mm and a calcium binding power of from 0 to 200 mg CaO/gm of anhydrous active substance measured at 22.degree. C.