The dehydration of diaspore to corundum was investigated by means of X-ray powder diffraction at reaction temperatures (400degrees and 600degreesC) as well as by transmission electron microscopy (TEM). The TEM studies were performed at the reaction interface of partially dehydrated natural diaspore crystals. The corundum produced consisted of thin dense regions which were separated by nanometer-sized pores forming lamellae with a periodicity of 3.7 nm. At the reaction front a transition phase (W) could be detected in electron diffraction patterns. The phase D' is isotypic to diaspore, but with a larger spacing of the close-packed (100) oxygen planes (a(D') from 0.475 to 0.480 nm). The expansion with respect to diaspore is explained by breaking of the hydrogen bonds of diaspore, considered to be the initial step of the transformation. The spacing of the lamellar pore system in corundum is explained by the misfit of the (100) planes of D' and the (0003) planes of corundum. We conclude that at well-fitting regions of the closed-packed planes at the D'/C interface, dense corundum is formed, while at misfitting regions, the formation of corundum is not favored and the pores are produced. Hence, the transformation of the solid phases takes place as a two-step process, i.e., D --> D' --> C.