There is interest in using Au-nanoparticle incorporated oxide films as functional sensor layers for high-temperature applications in optical-based sensors for measurements in both highly-oxidizing and highly-reducing atmospheres at temperatures approaching 900 degrees C-1000 degrees C because of a relatively high melting temperature combined with the inert nature of Au nanoparticles. This study includes a systematic series of experiments and theoretical calculations targeted at further understanding stability of Au-nanoparticle incorporated TiO2 films as archetype sensing materials. A combination of thermodynamic modeling and long-term exposure tests were utilized to unambiguously determine that gas stream composition-dependent reactive evaporation of Au (to form predominately Au(g) or AuH(g), depending upon the environment) at the surface of the nanoparticles is the dominant mechanism for mass loss of Au. Primary factors dictating the rate of reactive evaporation, and hence the associated film stability, were determined to be the gas stream temperature and the concentration of H-2, with the former playing a more significant role over the ranges of temperatures (700 degrees C-800 degrees C) and H-2 concentrations (1% to 29% H-2 by volume) explored. The mitigation of Au-mass loss through reactive evaporation was also successfully demonstrated by depositing a SiO2 overlayer on the Au-nanoparticle embedded films to prevent direct Au-nanoparticle/vapor-phase contact. (C) 2017 The Electrochemical Society. All rights reserved.