Boron suboxide thin films have been deposited on Si(100) and graphite substrates by reactive rf magnetron sputtering of a sintered B target in an Ar/O-2 atmosphere. X-ray photoelectron spectroscopy, elastic recoil detection analysis, Fourier transform infrared spectroscopy, x-ray diffraction, and transmission electron microscopy were applied to study the influence of the O-2 partial pressure on the film composition and microstructure. BOx, thin films with x=[0.02-0.21] and a C impurity of approximately 0.3 at. % were formed by varying the O-2 partial pressure from 7.2 x 10(-7) to 3.3 X 10(-2) Pa. All films were amorphous and the films with xgreater than or equal to0.15 contained boric acid on the surface due to a probable chemical reaction with water in laboratory atmosphere. Mechanical properties were evaluated by nanoindentation. As x was increased from 0.02 to 0.21, the elastic modulus decreased from 272 to 109 GPa. The change in the elastic modulus was attributed to the O concentration variations.