The transformation of gibbsite to boehmite in strongly caustic solutions was studied using quantitative X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy techniques. Under hydrothermal conditions, we identified two transformation mechanisms: (1) dehydration and in situ nucleation and (2) dissolution and nucleation. If the reaction container was not completely sealed, dehydration of gibbsite followed by in situ nucleation of boehmite was the preferred mechanism. Boehmite produced fibrous boehmite particles within the amorphous matrix of the decomposed gibbsite particles, which exhibited a poorly crystalline structure and a smaller size than the initial gibbsite particles. In a closed environment, the preferred mechanism was the dissolution of gibbsite along (001) planes. The final boehmite particles were not morphologically related to the initial gibbsite particles and could be many times larger than the gibbsite particles.