Thermal decomposition of boehmite and bayerite has been studied by X-ray diffraction and by NMR and IR spectroscopies. Coordination and site distortion of Al polyhedra have been estimated by Al-27 NMR spectroscopy while IR optical properties of transitional aluminas, eta-, gamma-, delta-, and theta-Al2O3 were derived from IR near-normal specular reflectance technique. In the low-temperature phases, aluminum vacancies are located in tetrahedral positions in gamma-Al2O3 while in eta-Al2O3 they are distributed at random between tetrahedral and octahedral positions. In addition, a small amount (5%) of pentahedrally coordinated aluminum was found in gamma-alumina, which has been tentatively assigned to Al at the external surface of alumina particles. At 800 degrees C a notable increase in structural ordering was detected in both phases; in the case of eta-Al2O3, this change accelerates the continuous transformation into theta-Al2O3 which is totally achieved at 1000 degrees C. On the contrary, gamma-Al2O3, does not show any sign of transformation until 900 degrees C; at 950 degrees C pentahedral cations disappear and formation of theta-Al2O3 is detected by IR and NMR techniques. From 900 to 1200 degrees C, gamma-, theta-, and alpha-Al2O3 coexist in samples obtained from boehmite. Above 1200 degrees C corundum is the only thermodynamically stable phase in both series of samples.