Abstract--The reaction of hexamethylene diisocyanate with the surface of heat-activatecl vermlcunte parallels the reactions with the oxides SiO2, A1203, MgO considered individually, which a number of authors have already described. The fixation ratio is about 5 molecules per 100 A 2 area. Nearly one half of the grafted molecules retain a "free" isocyanate function, which is requisite for a subsequent polyaddition of the processed mineral. The interaction with vermiculite occurs according to three principal processes; (i) formation of a surface urethane, (ii) formation of a surface carbamate, (iii) fixation by hydrogen bonds.