Thin films of nonstoichiometric silicon oxide (SiO x with x < 2) have been studied extensively during the past few decades because of their importance in many electronic and optoelectronic applications, and particular attention has been paid to models that can better describe their global structure. Herein, we present a detailed study of SiO x films deposited on silicon(111) and silica substrates using the low-pressure chemical vapor deposition (LPCVD) method by thermal oxidation of silane in an oxygen atmosphere at a temperature of 570 °C. The oxygen and silane flows in the reactor were varied to obtain films with different values of oxygen content x. Ellipsometry and m-line measurements were used to determine the complex refractive index of the deposited films. The oxygen contents in the films were measured by infrared spectroscopy, energy-dispersive X-ray spectroscopy (EDX), and time-of-flight elastic recoil detection analysis (TOF-ERDA). The oxygen contents in the films were also estimated from the measured values of the complex refractive indices using Bruggeman’s effective-medium aproximation (EMA). All of the results were in good agreement, except for those obtained from infrared spectroscopy, which corresponded to systematically higher oxygen contents. This effect was interpreted as being due to an inhomogeneous distribution of oxygen atoms in the films (phase separation). This issue was confirmed by X-ray photoelectron spectroscopy (XPS) analysis of the Si 2p core levels, which showed an almost-complete phase separation of the silicon-rich oxides into amorphous silicon and silicon dioxide, indicating that the mixture model is the most appropriate for the present films.