Optical and surface analysis techniques such as infrared spectroscopy, ellipsometry, and Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) have been used to study the way in which excess Si is incorporated in nonstoichiometric SiO2 films deposited by a low pressure chemical vapor deposition technique. It has been found that for this range of silicon content, the Si atoms in excess either cluster into a well-defined second phase embedded in a SiO2 matrix, or form a SiO2 type of material depending on the postdeposition thermal treatments given to the film. The oxides are deposited at a substrate temperature in the range of 650 to 750 degrees C using a mixture of N2O and SiH4 gases. The excess Si is introduced by reducing the nitrous oxide to silane gas ratio during the deposition (this ratio was in the range of 4 to 200). After deposition, a densification process consisting of a 1000 degrees C anneal in a nitrogen ambient for 30 min was given to some of the samples. The refractive index, as determined by ellipsometry, is sensitive to the excess of silicon in the films, although its behavior was similar for both annealed and as-deposited samples. The volume percent of excess silicon was estimated using spectroscopic ellipsometry for some samples. Infrared spectroscopy, on the other hand, shows a clear shift of the stretching vibration peak of Si-O-Si bonds toward lower wave numbers as the Si excess in the film is increased in the case of as-deposited films, while the annealed samples do not present this effect. Also, as-deposited samples show an absorption peak, at 890 cm(-1), that could be associated with partially oxidized silicon. AES and XPS spectra show a distinct difference in the peaks associated to Si bonded to either oxygen or another silicon atom for both types of samples indicating that at least a considerable part of the excess Si in the as-deposited samples is in the form of SiOx, while in the annealed samples it is mostly forming a second phase within a silicon dioxide matrix.