X-ray reflectivity (XRR) has been used to measure the thickness, density, and the surface and interface roughness of SiO2 and SiOF thin films prepared by plasma-enhanced chemical vapor deposition. The roughness values as obtained by atomic force microscopy measurements have been compared to those calculated from XRR data and a good agreement between the two techniques has been found. We have found that the density of SiO2 films increases with the N2O/SiH4 flow ratio (gamma) used for film deposition, approaching the value found in thermally grown films. Thus, films deposited with low g values have a low density; this situation holds also for substoichiometric oxides, suggesting that Si concentration is not fundamental in determining the film density. In this last case the film density is mainly determined by the presence of hydrogen, as demonstrated by the increased density found after an annealing process (400-600 degreesC). The density of SiO2 films can be increased by fluorine insertion; in the explored range (up to 4 atom % F), the SiOF film density increases almost linearly by increasing the fluorine concentration. This effect is reversible, since the loss of fluorine occurring during thermal annealing of these films determines also a decreasing of the density.