AlN films of thicknesses 670–780 nm were deposited on (1 1 1) silicon wafer, (0 0·1) sapphire, float glass and fused silica substrates by reactive mid-frequency (41 kHz) ac sputtering of dual hollow Al targets in a mixture of Ar and N2 gases. Nanostructured films of hexagonal wurtizite phase of AlN were prepared and characterized by UV-visible spectroscopy and X-ray diffraction. The novel geometry of dual target inverted cylindrical magnetrons produces very dense plasma and ion bombardment of the growing film, and facilitates the formation of highly crystalline AlN coatings without any deliberate substrate heating. Substrates showed significant influence on the crystal structure of AlN films; sapphire forms hexagonal AlN phase with very strong texturing toward (0 0·2) crystallographic orientation, silicon wafer grows small amount of metastable high-pressure cubic phase of AlN along with the predominant, thermodynamically stable hexagonal phase while the amorphous glass substrates form hexagonal AlN phase with a random orientation of the crystallites.