Water is ubiquitous, and its presence in the ambient humid air means that it may constitute an uncontrolled variable in chemical processes. Methods for humidity control may involve complete removal of water and its vapor by procedures such as evaporation under vacuum, use of drying agents such as silica gel, adjustment to a desired humidity by use of saturated aqueous solutions, or adjustment to a desired humidity by use of a pair of related salt hydrates (such as CuSO4 center dot 3H(2)0 + CuSO4 center dot 5H(2)O). By the phase rule, the presence of three phases at a fixed temperature (in the latter two cases) ensures a constant equilibrium humidity. The thermodynamics of these chemical means of humidity control is presented, together with a database of almost 300 salt hydrate pairs which may be considered for humidity control by mixing into the reaction medium. However, the possibility of interaction of the hydrates with the reaction medium should not be neglected.