Na-β''-gallate/water vapor reactions have been studied via weight gain, X-ray diffraction and electron microscopy. Two hydrates form and their appearance is kinetically controlled by twofold diffusion. Na-β''-gallate hydrates and the dissolution of Na 2 O leads to hydronium-β''-gallate. The latter decomposes irreversibly to Ga 2 O 3 at high temperatures. Structural modifications involved in hydration and dehydration are discussed.