Undoped oxygen-deficient SnO x films were deposited on Si (100) and BK7 glass substrates by the reactive ion-assisted deposition technique. The average impinging energy ( E a) of the oxygen ion on depositing a Sn atom, a relative arrival ratio of ion to atom ( Γ), was varied from Γ = 0.025 ( E a = 25 eV atom −1) to Γ = 0.1 ( E a = 100 eV atom −1) by controlling the discharge voltage at a fixed ion-beam potential of 500 V. Crystalline structures of as-deposited SnO x films appeared to be amorphous for all E a values. In order to precisely examine the dependence of oxidation from SnO to SnO 2 on initial oxygen content and annealing temperature, as-deposited SnO x films containing initially different oxygen contents were annealed at 400 ∼ 500 °C in a low vacuum (∼5 × 10 −3 Torr) for 1 h to remove the external oxygen diffusion effect during the annealing process. The heat-treated SnO x films at 400 °C showed polycrystalline SnO structure until E a = 50 eV atom −1 and were still amorphous over E a = 75 eV atom −1. But after annealing over 500 °C those films over E a = 75 eV atom −1 exhibited diffraction peaks at (110), (101), and (211), characteristic of polycrystalline SnO 2. Scanning electron microscopy (SEM) micrographs for the as-deposited SnO x films show grains as small as a few tens of ångstroms in size, but those after 500 °C annealing reveals that the number of granular grain became larger in cross-sectional SEM and the grain grew to 300 Å at E a = 100 eV atom −1. For as-deposited films, the root-mean-square of surface roughness ( σ) slightly increased from 9 to 25 Å as E a was increased, but conversely it reduced from 36 Å to 13 Å after annealing. From quantitative Auger electron spectroscopy, it was observed that characteristic transitional Auger peaks of Sn MNN shift to lower kinetic energy by as much as 3.8 ∼ 4.2 ± 0.02 eV in the case of the SnO films and 4.8 ∼ 5.2 ± 0.02 eV for SnO 2 films, respectively, with respect to that of Sn metal (Sn 0). Based on the valence band spectra taken by using He I angle-resolved ultraviolet photoelectron spectroscopy (angle-resolved UPS), two different phases of polycrystalline SnO and SnO 2 films were clearly distinguished. The binding energy difference between the first peaks derived from SnO and SnO 2 films in angle-resolved UPS spectra below the Fermi level was 1.6 eV. This value is very close to the value obtained from clean SnO and SnO 2 samples by previous valence-band XPS spectra. As-deposited and annealed films showed optical transmittance as high as more than 80% grown at higher than E a = 50 eV atom −1 in the visible wavelength and refractive index close to the bulk SnO 2 value for the film deposited at E a = 100 eV atom −1 after annealing.