Abstract: A characteristic feature of the present invention is that the crosslinking agent comprises grains, each grain having a nucleus consisting of a substance inert to the substance forming crosslinks with hydroxyl-containing water-soluble polymers, and an external layer surrounding said nucleus and containing compounds of tetravalent metals selected from the group consisting of titanium, zirconium, hafnium and tin.The particles for the chemical treatment of hydroxyl-containing water-soluble polymers can be prepared by applying a substance capable of forming crosslinks with the hydroxyl-containing water-soluble polymers on inert, finely dispersed particles and by hydrolytically treating this substance so as to form an external layer of the grain.