Environmental barrier coatings (EBCs) are used in commercial turbine engine applications as protection for ceramic matrix composites, yet the high-temperature water vapor reaction mechanism for EBC materials is not fully understood. Here, the water vapor reaction mechanism for barium strontium alumino-silicate (BSAS), an early generation EBC candidate, was determined from the time and temperature dependences of material loss. BSAS water vapor exposures were performed at 1200 °C, 1300 °C, and 1400 °C for 24, 48, and 72 h, at maximum gas velocities of ~ 240 m/s. FactSage thermodynamic calculations were shown to support the experimental findings, where the steam reaction mechanism consisted of volatilization of all BSAS oxide constituents as gaseous metal hydroxide species, i.e. Ba(OH)2, Sr(OH)2, Al(OH)3, and Si(OH)4 (g).