The oxidation of Si in 15 mbar isotopically labeled O-2/H2O-gas mixtures at 750 to 1000 degrees C was studied in situ by probing the gas-phase composition of an enclosed volume with a mass spectrometer. The experiments reveal interactions between the gas phase and the oxide which are not accessible from analyses of the solid reaction product. The dissociation rate of O-2 was calculated from oxygen exchange reactions. O-2 reversible arrow O-2 and O-2 reversible arrow SiO2, and compared with the oxidation rate. In the O-2 + H2O-gas mixture, the dissociation rate of H2O was calculated from the O-2 reversible arrow H2O oxygen exchange and compared with the oxidation rate. In the oxidation process, the dissociation rate of H2O decreased by a factor of more than 20. A general discussion on the dependence of the dissociation rate on the oxidation rate is made in relation to different types of migrating species in various systems.