The gaseous species SiO(OH) and SiO(OH)(2) were detected as products of the reaction of SiO2(1) 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 Delta(f)H(298)degrees 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 Delta(f)H(298)degrees 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.