In the solid state, AG-013958 exists as a three-dimensional hydrate that dehydrates completely at 109 degrees C with subsequent melting at 183 degrees C. Water molecules in the AG-013958 crystal structure occupy hydrophilic sites and anchor neighboring drug molecules through H bonding. Because hydrates/dehydrates display different physical properties related to dissolution, solubility & bioavailability, the stability of AG-013958 monohydrate was investigated. A reversible hydration/dehydration of AG-013958 was observed in which a meta-stable dehydrate was created on removal of H2O. Solid-state x-ray diffraction data suggest the reversible hydration mechanism results from molecular recognition between drug molecules, which affects molecular packing and the observed channel network. The hydration rate was largely dependent upon vapor pressure and the rate of hydration doubled from 7.6 to 15% R-H at ambient conditions, whereas temperature influenced the hydration kinetics to a much lesser extent. Hydrate stability can be controlled by maintaining pharmaceutical processing conditions at R-H >= 15% at 25 degrees C.