The growth of anodic oxides of crystalline Si in an O2 plasma has been studied. The kinetics are found to follow those expected when neutral O atoms in the plasma capture an electron at the substrate surface and diffuse through the growing oxide under the influence of an applied positive electric field. The oxides have been characterized using infrared spectroscopy, glancing incidence x-ray reflectometry, and electric capacitance/voltage measurements. Shifts to lower wave numbers of the transverse and longitudinal optic infrared modes associated with the asymmetric stretch of the bridging O’s with respect to thermally grown oxide values are observed. We conclude that these shifts result from structural modifications induced by ultraviolet photons present in the O plasma. A simple model suggests that the plasma grown oxides have a density ∼3.8% larger than thermal SiO2 and an average Si–O–Si bridging bond angle ∼1.6° smaller. The plasma grown oxides are inhomogeneous in the volume, the bond angle shift increasing to ∼−2.3°, and the density decreasing to +2% with respect to thermal oxide near the Si substrate/SiO2 interface. X-ray reflectivity data, which are consistent with film inhomogeneity, enable us to determine the film thickness and surface (air/SiO2) roughness. The latter is always ⩽1 nm. Electrical measurements suggest that the oxides contain fixed oxide charges and interface states which can be reduced by low temperature annealing. The leakage currents remain high, however.