The structural properties of superlattices composed by hydrogenated amorphous silicon/silicon carbide (a-Si:H/a-Si1-xCx:H) and silicon/germanium (a-Si:H/a-Ge:H), deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique, were analyzed by means of small-angle X-ray diffraction. The relevant structural parameters, such as the multilayer period, the individual layer thickness, the width of the interface and the optical constants, were determined by modeling the experimental reflectivity. The model was based on the dynamical diffraction theory, including material mixing at the interface, interface roughness and random variation of component thickness. In addition, the effect of the direct beam and background on the measured intensities were considered.