Transformation of Al(NO 3 ) 3 ∙9H 2 O (upon heating in the range of 20–1200 °C) into blends of amorphous and crystalline boehmite (210–525 °C), amorphous alumina and crystalline γ-Al 2 O 3 (850 °C), and crystalline α-Al 2 O 3 (1100 °C) was analyzed using X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), infrared (IR) spectroscopy, thermogravimetry, and low-temperature nitrogen adsorption. Boehmite consists of nanoparticles of 6–10 nm in diameter, and part of them has crystalline structure observed in HRTEM images, despite they are XRD amorphous. The nanoglobules are surrounded by amorphous aluminum hydroxide with chains of –AlO(H)–O–AlO(H)– of 1–5 nm in length. Heating of samples at 350–525 °C gives mesoporous aluminum hydroxide with a relatively narrow pore size distribution. An increase in calcination temperature to 850 °C decreases the porosity of alumina composed of amorphous and crystalline (γ-Al 2 O 3 ) phases. Calcination at 1100 °C gives α-Al 2 O 3 with strongly decreased porosity of aggregates.