Boehmite (gamma-AlOOH) is often the initial product in sol-gel syntheses of high surface-area alumina powders. Depending on the processing routes, the microstructure of an AlOOH powder can be tailored to exhibit nanometer particle size and complex interfacial OH sites. Neutron small-angle scattering (SANS) and spectroscopy were utilized to characterize the microstructure and hydrogen vibrations of a nanostructured pseudoboehmite and a highly crystallized boehmite powder. The SANS data reveal a mass-fractal-like microstructure in pseudoboehmite. The vibrational bands of hydrogen atoms in pseudoboehmite are broad, which is indicative of a high degree of proton delocalization and disordered OH sites. (c) 2004 Elsevier B.V. All rights reserved.