The mechanisms of oxygen exchange between thermal silicon oxide films and carbon monoxide have been studied using 18O as an isotopic tracer. SiO2 layers of natural isotopic composition, obtained by thermal oxidation of silicon, were exposed at 1100 °C to 13C18O gas at pressures ranging from 50 to 350 mbars. 18O concentration depth profiles were determined using the nuclear narrow resonance profiling technique with the narrow resonance near 151 keV in the reaction 18O(p,α)15N. 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.