This study is focused on solving the crystal structure of E6070, a novel IKKβ protein kinase inhibitor, and characterizing its solid state. E6070 exists as a stable crystalline trihydrate (hydrate I) that undergoes two dehydration events on heating. Neither crystal form nor water content changed under the relative humidity range from 11% to 93% at 25 °C. Crystal transformation to hydrate II occurred in the first dehydration process beginning at ˜70 °C followed by loss of almost all remaining crystal lattice water as heating continues up to 160 °C. The observed weight loss of the first step was significantly higher than two equivalents of water. The X-ray powder diffraction proved that the anhydrate is an amorphous form that reversibly reforms the original trihydrate when exposed to atmospheric moisture. Hydrate II more readily forms hydrate I than from the anhydrate. The activation energy of the second step accompanied with amorphism was approximately 1.5 times greater than that of the first. In addition, a gradual decline of activation energy depending on the extent of dehydration was observed, implying reversible conversion in both steps. Therefore, the trihydrate was considered suitable for further development as its solid state is controllable during the manufacturing processes and the shelf life.