The article presents results on the incorporation of starch in a poly (vinyl acetate) matrix by co-grinding. This treatment permits a good dispersion of the filler in the matrix and favors agglomeration phenomena which limit segregation and enhance interactions between the two materials. The composite materials offer interesting water resistance properties. Water absorption phenomena were studied and it was shown that water penetration in the materials is controlled by diffusion. The diffusion coefficients of water were determined for the matrix, the filler and the mixture and they are respectively equal to 3.3 10-11, 1.1 10-10 and 5.5 10-11 m2/s.