Thermodynamic calculations of mineral stability fields under a range of predicted surface conditions for Mars indicate that goethite will be unstable except under the wettest and coldest parts (pH2O ∼ 10−6 atm, T < 200°K) of this range. If the planetary atmosphere is strongly reducing, as implied by some estimates of CO abundance, the measured water vapor pressure given above may be local or transient and would be out of equilibrium with the atmosphere in general. This would imply that nearly any hydrous mineral would be unstable on the Martian surface. FeCO3 would then be stable over Fe2O3 or FeO·OH. The high CO2 pressure of the Martian atmosphere will stabilize the carbonates of Mg and Ca in weathering processes. The aluminum that is not bound in feldspars would appear as Al2O3 (or AlO·OH) in weathering products. The presence of some carbonates may explain the ∼3.45-μ absorption band of Sinton's 1956 and 1958 infrared spectra. No band is found, however, for the 1.05-μ absorption band of siderite. If the atmosphere of Mars is still in an early stage of development and the planet is relatively undifferentiated, small credence should be given to the pre-existence of abundant water on the planet. Accordingly, theories that account for abundant hydrated minerals as ‘fossil’ forms must still stand the test of producing the proper conditions for formation of hydrous phases.