The mass transport during wet oxidation of Ti34Si23N43 films at 500, 600 and 800 degrees C was studied by monitoring the position of a buried platinum marker. The oxide layer thickness and the position of the platinum marker as a function of the oxidation temperature and duration were monitored by 2.0 MeV He-4 backscattering spectrometry. At all temperatures, the oxidant diffuses inward and nitrogen outward through the oxide scale with respect to the stationary Ti-Si matrix.