The elastic properties of thin graphitic carbon and hexagonal boron nitride films have been determined by Brillouin light scattering from thermally excited surface acoustic phonons. The carbon films have been grown by plasma beam deposition at 420 degrees C. The BN films have been deposited by r.f.-magnetron sputtering and simultaneous ion plating. In the case of amorphous carbon a layered substructure has been found with a rather compliant intermediate layer at the interface. The stiffness of the films increases with increasing ion energy. As for carbon the BN coatings exhibit a pronounced elastic anisotropy indicating a preferred orientation of the hexagonal plants perpendicularly to the surface. The comparison of the Young's moduli determined by load indentation and by Brillouin light scattering allows the conclusion that the material is composed of two or more components of different stiffness.