Adsorption properties of U.I.C.C. Rhodesian chrysotile and crocidolite were tested in aqueous solution with acid and basic dyes (Eosine Y. Picric Acid, Acridine Orange and Methylene Blue). Rates of adsorption showed a diffusion of only basic dyes in micropores of fibers. The strong adsorption of basic dyes on the surface of the fibers led us to suppose that the active sites have a negative charge. Our hypothesis was supported by the verification of an increase in adsorption capacity whick depends on depletion of Mg++ from chrysotile and of Fe+++ from crocidolite.