X-ray reflectivity was used to measure the coefficient of thermal expansion (CTE) and the density profile of hydrogen silsesquioxane thin films deposited on silicon wafer substrates. This study has demonstrated that this technique is capable of measuring the thickness change of 40 nm thick films with a CTE in the 10(-5)/degrees C range. The CTE of ultra-thin hydrogen silsesquioxane films was found to be about 40 ppm/degrees C from 20 to 175 degrees C. After-taking the substrate constraint into consideration, the calculated CTE of the films in their unconstrained condition was about 20 ppm/degrees C. In addition, the density of the films was found to be smaller than that of silica and was non-uniform along the thickness direction with a low density region adjacent to the hydrogen silsesquioxane/silicon interface.