The thermochemical stability of Y2Si2O7 was assessed in a high-temperature high-velocity water vapor environment to improve the understanding of the mechanisms that lead to SiO2 depletion. Spark plasma sintered Y2Si2O7 specimens were exposed in a steam-jet furnace at 1000 degrees C and 1200 degrees C for 3-250 hours, steam velocities of 131-174 m/s and at 1 atm H2O pressure. These exposures resulted in the selective volatilization of SiO2 to form volatile Si(OH)(4) and porous Y2SiO5. Microstructural evolution from fine rectangular pores at short times to larger rounded pores at longer times was observed. Mechanisms contributing to the overall depletion reaction kinetics were evaluated and include the interface reaction to form Y2SiO5 and Si(OH)(4) (g), Y2SiO5 coarsening, development of tortuosity in the pore network and diffusion of H2O (g) and Si(OH)(4) (g) through pores by molecular diffusion and/or Knudsen diffusion. SiO2 depletion was found to follow parabolic volatilization kinetics (k(p) = 0.38 mu m(2)/h) at 1200 degrees C indicating the reaction is limited by a diffusion process, most likely the outward diffusion of Si(OH)(4) (g) through pores. Results are utilized to assess the viability of Y2Si2O7 and other rare-earth silicates as environmental barrier coating (EBC) materials for SiC ceramic matrix composites (CMCs).