The hydrated crystal structures of cromolyn, cefazolin, and fenoprofen sodium salts are reported. The former two compounds are non-stoichiometric hydrates, whereas the fenoprofen lattice maintains its stoichiometry over a broad range of relative humidity. The relationship between composition, lattice parameters, and relative humidity is studied using a combination of moisture sorption isotherms and variable humidity X-ray powder diffraction. The dehydration properties of the sodium salts are related to the ion coordination and hydrogen bonding of the water molecules in the structures. Anisotropic lattice contraction is observed during dehydration of the cromolyn and cefazolin sodium and is related to the closeness of intermolecular contacts in the hydrated structures.