The X-ray reflectivity (XRR) technique was used to study monocrystalline silicon samples implanted with H-2 ions at an energy of 31 keV and to the dose of 2 x 1016 hydrogen atoms/cm(2). All samples were subsequently isochronally annealed in vacuum at different temperatures in the range from 100 to 900 degrees C. Although the hydrogen depth distribution was expected to be smooth initially, fringes in the XRR spectra were observed already in the implanted but not annealed sample, revealing the presence of a well-defined film-like structure. Annealing enhances the film top to bottom interface correlation due to structural relaxation, resulting in the appearance of fringes in the larger angular range, already at low annealing temperatures. The thickness of the film decreases slowly up to 350 degrees C where substantial changes in the roughness are observed, probably due to the onset of larger clusters formation. Further annealing at higher temperatures restores the high correlation of the film interfaces, while the thickness decreases with the temperature more rapidly. (c) 2006 Elsevier B.V. All rights reserved.