The gaseous species SiO(OH) and SiO(OH)2 were detected as products of the reaction of SiO2(l) and H2O(g) near 2000 K, using effusion-beam mass spectrometry. No other silicon-containing hydroxides or oxyhydroxides were observed. From third law analysis of equilibrium data, values of Δ[sub f]H°[sub 298] of SiO(OH) and SiO(OH)2 were evaluated as -118 and -213 kcal mol-1, respectively, both ±4 kcal mol-1. Several equilibrium measurements on gaseous SiO2 yielded a Δ[sub f]H°[sub 298] value of -67 kcal mol-1, which is 6–10 kcal mol-1 less stable than values given in most compilations. It appears that the new thermochemical data for SiO(OH) and SiO(OH)2, plus a literature estimate for Si2O(OH)6, can be used to model the equilibrium behavior of the silica–steam system over a wide range of temperatures and pressures. [ABSTRACT FROM AUTHOR]