We use dynamic secondary ion mass spectrometry (SIMS) to examine the mechanism of H (D) incorporation into and retention within a buried SiO2 film at 625 degrees C. We find that diffusion of H-2(D-2) through the Si/SiO2/Si structure at this temperature is facile and that isotopic exchange occurs at the interfaces upon subsequent forming gas anneals at 625 degrees C. A detailed examination of the isotopic exchange process indicates that the interfaces do not exhibit equivalent behavior. We also describe the artifacts observed in the SLMS profiles by comparing positive and negative secondary ion profiles.