The mechanisms of oxygen exchange between thermal silicon oxide films and carbon monoxide have been studied using O-18 as an isotopic tracer. SiO2 layers of natural isotopic composition, obtained by thermal oxidation of silicon, were exposed at 1100 degrees C to (CO)-C-13-O-18 gas at pressures ranging from 50 to 350 mbars. O-18 concentration depth profiles were determined using the nuclear narrow resonance profiling technique with the narrow resonance near 151 keV in the reaction O-18(p,alpha)N-15. The results show that oxygen exchange takes place via two distinct processes and a mechanism for each process is proposed in the present work. The diffusion coefficient of CO molecules in the silica and the oxygen exchange frequency between CO and the silica are also determined.