The application of the ion-assisted deposition (IAD) technique to the evaporation of SiO thin films has been studied. SiO thin films 2700 and 540 nm thick were evaporated on silicon and sapphire substrates, with and without low energy (120 eV) ion bombardment. Transmittance spectra in the IR region of 1000-5000 nm show that ion assistance has induced a clear change in the refractive index of the films. In addition, a new absorption peak at 2900 nm appears in the IAD-grown samples, the intensity of this peak being dependent on the film thickness. This different optical behaviour has been related to an IAD-induced change in the stoichiometry of the films. Auger depth profiling of the SiO films indicates a decrease of the Si/O Auger intensity ratio by a factor of 2 in the treated samples. This variation implies a change in the atomic concentrations of oxygen and silicon in the IAD-treated films. The results presented in this work are very promising for application of the IAD technique to the design of multilayer IR filters.