A method for functionalizing the surface of a substrate, performed in a supercritical fluid medium, may include: (i) providing a substrate having labile hydrogen functions on the surface; (ii) bringing the substrate into contact, in a supercritical fluid, with at least one organic molecule carrying at least one blocked isocyanate function which can be activated by heating; and (iii) subjecting the whole to a temperature sufficient to bring about the release of the blocked isocyanate function carried by the molecule, and the covalent grafting of the molecule by reaction of the isocyanate function with a labile hydrogen function on the surface of the substrate.