The structural properties of hydrogenated amorphous silicon/germanium superlattices (a-Si:H/a-Ge:H), deposited by the plasma enhanced chemical vapor deposition method, were analyzed by means of small angle X-ray diffraction. The multilayer period and the individual layer thickness, as well as the width of the interface, were determined. The periodicities and interface widths were obtained for samples deposited on different substrates. The experimental reflectivity was compared to theoretical simulations, considering interface roughness, material mixing and non-homogeneous thickness. The thermal stability of the bilayer components was studied by means of heat treatments. A crystallization process occurs after the diffusion of the materials, being dependent on the thickness of superlattice components. The crystallization happens at temperatures around 600°C and is retarded for structures with smaller layer thickness.