The physical properties of aluminum oxyhydroxide (AlOOH) samples of various sources have been characterized with techniques of X-ray diffraction (XRD), derivative thermogravimetry (DTG) and electron microscopy (EM), and correlated to their pore size distributions monitored with physisorption of nitrogen. Three different crystallinities, i.e., amorphous-boehmite, pseudo-boehmite and boehmite, have been distinguished from the investigated AlOOH samples according to results of XRD and TG. The EM studies further provided information on the primary and the aggregated structures of these samples. These primary particles retained their respective structures and had a profound effect on the pore size distribution of gamma-Al2O3 samples converted from a phase transformation of these AlOOH samples upon calcination at temperatures above 400 degrees C. A further calcination at T > 1200 degrees C transformed the gamma-alumina into alpha-alumina. The latter transformation caused a drastic sintering of all kinds of primary particles to form vermicular solid crystals. The rate of sintering during the gamma to alpha transformation increased with the calcination temperature but decreased with the crystallinity of primary particles in the original AlOOH samples.