Microcrystalline boehmites were synthesized by aging amorphous precursors derived from base hydrolysis of aluminum nitrate solutions at 85 degrees C. The susceptibility of the resultant boehmite gels to form a colloidal dispersion by reaction with dilute nitric acid, i.e., their peptizability, was determined based on their particle size measured by dynamic light scattering. Three different classes of peptizabilities were obtained by modifying parameters during the neutralization step and were related to the characteristics of the boehmite gel and its precursors before aging. X-ray diffraction, thermal analysis, solid state Al-27 NMR, and transmission electron microscopy techniques were applied to the characterization of the materials. The preparation of peptizable boehmites was favored by low temperature hydrolysis where the amorphous AIO(OH) precursor, after aging, readily converted to boehmite crystallites that did not aggregate into secondary particles. Nonpeptizable boehmites were obtained at hydrolysis ratios (OH/AI > 3) at high temperature, leading to boehmite crystallites which condensed to large strongly interconnected polycrystalline fibrils. Partly peptizable boehmite was obtained at high temperatures but lower OH/AI ratios. Under this condition, the fast crystallization of boehmite in the neutralization step was retarded, thus hindering strong intercrystallite aggregation. This could be correlated to the presence of a significant portion of tetrahedrally coordinated aluminum in the precursor. A brief discussion on the mechanisms of formation of the aluminum gel precursors is also presented.