X-ray reflectivity has been used to study properties of thin SiO2 films on (100) Si prepared by plasma-enhanced chemical vapor deposition (PECVD) or by anodic oxidation. Reflectivity curves were analyzed to obtain information concerning the thickness, density (p) and interface roughness (σ) of these oxide films. We found that the PECVD films have typically a density of 0.98 psi with a sharp Si/SiO2 interface (σ < 0.2 nm) and a surface roughness of -0.5 nm. Anodic oxide films have a lower density (p = 0.88 psi) and larger interface and surface roughness σ - 0.5 nm. High temperature annealing of the anodic oxides up to 900 °C resulted in no appreciable densification but caused an increase in the interfacial roughness. Considerable variations were observed in the reflectivity curves from a PECVD oxide film annealed in the 650-1050 °C range.-Modeling of these results suggests the existence of a thin interfacial layer (-0.5 nm) of less density (∼0.9psi) at the original silicon/PECVD oxide interface.