High-temperature, high-velocity water vapor (steam-jet) exposures were conducted on Y2O3, Y2SiO5, Y2Si2O7, and SiO2 for 60 hours at 1400 degrees C. Volatility of Y2O3 was not observed. Phase-pure Y2SiO5 exhibited SiO2 loss forming Y2O3 and porosity. A mixed porous and dense Y2SiO5 layer formed on the surface of Y2Si2O7 due to SiO2 depletion. The mechanisms and kinetics of the reaction between SiO2 and H2O(g) to form Si(OH)(4)(g) from Y2SiO5, Y2Si2O7, and SiO2 are discussed.