Abstract: Silicon oxide thin films deposited by reactive ion-beam sputtering (RIBS) of Si targets in Ar/O2 producer gas mixture were investigated. The refractive index n, extinction coefficient k, and stoichiometry were dependent on the oxygen concentration in Ar/O2 producer gas mixture and the deposition rate. Silicon oxide films with refractive index of n =1.48 and extinction coefficient of less than 10-5 were deposited with the RIBS method at the wavelength of 0.63 m. An increase in the deposition rate results in the growth of refractive index and extinction coefficient of the film due to incomplete oxidation of silicon and formation of suboxides. In the case of deposition rate more than 0.3 nm/s, even if sputtering is performed in pure oxygen environment, the minimal value of refractive index is more than 1.5. The ana-lysis of the IR transmission spectrum proves that all the deposited films have oxygen deficiency, and the stoichiometry of silicon oxide does not exceed 1.8 based on the investigation of the position of the main absorption peak. The dependence between IR adsorption peak position and refractive index of silicon oxide is established. Thus, depositing silicon-oxide thin films with RIBS without heating the substrates limits the increase of deposition rate.