Co-evaporated Au-Ti thin films deposited onto sapphire and silicon dioxide substrates were thermally treated over a temperature range of 300–900°C in both vacuum and reactive amb ients. Vacuum annealing produced negligible reaction between the oxide substrate and film, the Au-Ti, however, reacted to form intermetallic compounds with little net elemental redistribution in the film. Heat treating in oxygen resulted in segregation of titanium to the surface where it formed both rutile and anatase structures. In addition, the high interfacial energy between the gold and the oxide phases led to the formation of large (5 micron diameter) gold particles on the film surface or voids at the titania-gold interface. Annealing in Ammomia produced an oxynitride surface in addition to the Ti-Au separation, Au particulate formation, and/or interfacial voiding already observed. The extent of surface degradation observed during these reactive ambient anneals varied with Ti concentration and temperature. The film did not react with SiO2 under any circumstances, however, at temperatures above 650°C in reactive ambients the titanium accumulated at the Al2,O3 interface. These results were obtained by RBS, X-ray diffraction, TEM and SEM.