The diffusion process Of MoO3 and ZnO on stable oxide thin films (SiO2, Al2O3, TiO2) was investigated mainly by means of synchrotron-radiation-excited total reflection x-ray fluorescence spectroscopy (SR-TXRF). A stripe Of MoO3 or ZnO on the oxide thin film was used as the diffusion source. After thermal treatment, MoO3 diffused onto the surface of those films and formed a monolayer or a submonolayer. The diffusion capacity and the diffusion rate Of MoO3 on each film differed significantly. Sublimation of MoO3 also was detected during the diffusion process. A possible explanation for all the phenomena is the combination of surface diffusion onto the surface of the support and transportation via the gas phase. By contrast, ZnO hardly diffuses onto the surface of the film due to its high melting point. Copyright (C) 2001 John Wiley & Sons, Ltd.