Sulphur Springs, located near the western edge of the Valles caldera, north central New Mexico, is the only vapor-dominated surface manifestation of any significance found in the Valles. The percolating acid sulfate waters and fumarolic gases have leached and oxidized the county rock near the hot springs to a bleached wasteland. The near-surface mineral assemblages at 90¿C are restricted to one and two phases occurring in contact with a variety of solutions and gases. The conditions of local equilibrium at 90¿C are approximately halotrichite+alunogen, log FO2 -45, log fS2 -12, pH 0.3; szomolnokite+pyrite, log fO2 -46, log FS2 -16, pH 0.8; alunite, log FO2 -50, log fS2 -10, pH 1-2; kaolinite+sulphur, log fO2 -53, log fS2 -10, pH 1 2, log aK+=0.0012 m, log aSiO2=0.0033 m; and pyrite, log fO2 -60, log fS2 -19, pH 4.3. This study has demonstrated that local equilibrium is closely approached even under the low-temperature and -pressure conditions found near the surface.