Protective coatings for refractory alloys are typically based on silicides, borosilicides, and aluminides. In high temperature turbine and boiler environments containing substantial amounts of high temperature water vapor, these materials will oxidize to from silica, boria, and alumina. In addition, gaseous hydroxides of boron, silicon, and aluminum are sufficiently stable that volatilization of the coatings is a life-limiting degradation mechanism for long term application of these coatings. An understanding of the stability of these oxides in high temperature vapor is essential for coating lifetime predictions. In this paper, the stability of silica, boria, and alumina, as well as the stability of the base molybdenum and niobium metals in high temperature water vapor will be reviewed. Thermochemical data for gaseous metal hydroxides as well as methods for determining thermochemical stability of the oxides and base metals will be described. Kinetics of the volatilization reaction will be discussed.