In this paper the high-temperature corrosion of Al2O3 refractories by MOH(g) (M = Na, K) found in the combustion atmospheres of typical air- and oxygen-fired glass-melting furnaces is examined using thermodynamic equilibrium calculations. These hydroxide species are considered to be the primary reactive alkali species since their partial pressures are significantly larger than those of M(g), the next most abundant gas-phase alkali-containing species expected in typical furnace atmospheres. Thermochemical simulations show that corrosion of alpha-alumina by NaOH(g) at typical furnace p(NaOH(g)) of around 200 ppm under oxy/fuel-fired conditions is unlikely as long as the refractory temperature exceeds 1564 K. For KOH(g) at 200 ppm, the temperature of the refractory must exceed 1515 K to avoid corrosion. Under air-fired conditions, p(NaOH(g)) is considerably lower (40-80 ppm); at 50 ppm, corrosion is thermodynamically unfavorable at temperatures above 1504 K. For KOH(g) at furnace levels of similar to50 ppm, temperatures must be above 1458 K. The paper also presents a re-evaluation of the thermodynamic and phase equilibrium properties of the Na2O-Al2O3 and K2O-Al2O3 binary systems to develop accurate and self-consistent thermodynamic data. The data for MAl9O14 (beta-alumina) and M2Al12O19 (beta"-alumina) are particularly critical since these phases are likely products of the corrosion of alumina refractories by MOH vapors in glass melting furnaces.