The mechanisms whereby solid oxides can react with hydrogen and water are discussed. The potential range in which lithium-containing phases are stable in water can be calculated using the Gibbs triangle. Examples from the literature are cited. Materials with potentials above the water stability window will tend to react with water vapor in the atmosphere, absorbing protons and electrons. High potential solids can also be used to extract lithium from aqueous solutions. Experiments on the effect of the exposure of Pr6O11 to water on the insertion of lithium are reported.