Rare-earth silicate compounds, such as those in the Y2O3-SiO2 system, are promising candidates for coatings of SiC-based ceramics and ceramic matrix composites in combustion environments. The predicted lower activity of silica in these silicates will lead to less reactivity with the water vapor combustion products. A procedure for measuring silica activities in this system is discussed. Knudsen effusion mass spectrometry is used and the measured vapor pressure of SiO(g) is correlated to activity. Due to the low vapor pressure of SiO(g) in the temperature range of interest, a reducing agent is utilized to boost this vapor pressure without altering the solid-state composition. In addition, corrections are made for nonequilibrium vaporization. The measured silica activities are: Y2O3+Y2O3 center dot SiO2 two phase field: log[a(SiO2)]=-5200.26(1/T)+0.0567 (1532