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Comprehensive X-ray reflectivity (XR) studies were conducted to characterize the structure of thin polyalkylsilicate films made of a poly(methylsilsesquioxane-co-ethylenylsilsesquioxane) precursor containing a star-shaped poly(ε-caprolactone) as a pore generator (porogen). The films were deposited on silicon wafer substrates by spin-coating and subsequently cured at various temperatures. Such spin-on glasses have a potential application as a low-dielectric-constant material for advanced semiconductors. Because high-intensity synchrotron X-ray radiation was used, the XR data could be measured over 9 orders of magnitude in intensity, which facilitated the observation of fine structural details. A hierarchical fitting procedure for modeling the XR data is given. By evaluation of the critical angle of total reflection of the film material, which was observed at smallest angles, in particular the film electron density could be determined with a high accuracy. The films cured at 420 °C show a lower electron density as compared to those cured at 250 °C. This is explained by the fact that at the higher temperature the porogen is calcined and escapes from the films leaving behind a nanoporous structure. Film porosities could be estimated from the observed changes in the electron densities. From the very large number of high-frequency oscillations observed in the XR curves, it is concluded that the films exhibit a homogeneous, well-defined structure with small interface and surface roughness. The film thickness could be determined with an accuracy of ±1 Å. The observation of an additional low-frequency modulation of the XR curves revealed a surface skin layer with a thickness of ca. 45 Å and with a slightly increased electron density as compared to the bulk of the film.