Thin metallic (Al, Mo, Ti) and ceramic (Al2O3) layers, prepared by the ion beam assisted deposition, were investigated with the aid of the X-ray reflectivity measurement. A procedure based on the Fourier transformation of the reflectivity curves was applied to resolve the real structure of coatings, namely to submit the number of subsequent layers, their thickness, electron density and roughness. The final refinement of the free parameters of the respective structure model was carried out using the non-linear least-squares algorithm. The applicability of the ab initio method and the stability of the refinement are discussed.