The present invention relates to a composition comprising at least one masked or unmasked polyisocyanate in the form of an aqueous emulsion whose mean size is between 0.1 and 10 micrometers; and at least one polyhydroxylated polymer, advantageously a polyol, in the form of a second aqueous dispersion whose mean size is between 10 and 200 nanometers wherein the content of polyhydroxylated polymer, advantageously of polyol, expressed in mass percent, is at least equal to R-20, with R equal to the ratio of the mean size (d.sub.50) of the emulsion to that of the dispersion of polyhydroxylated polymer, advantageously of polyol.