By means of physical vapor deposition, a variety of metastable phases can be deposited due to the low substrate temperatures. Results are reported on the preparation of homogeneous metastable Al-O-N layers produced by reactive magnetron sputtering ion plating using an Al target and an Ar-O-2-N-2 gas mixture. At 190 degrees C substrate temperature and variable gas pressures of O-2 and N-2 a variety of coatings with different O and N content were prepared and analyzed by means of X-ray photoelectron spectroscopy, Auger electron spectroscopy, high-resolution transmission electron microscopy and high-resolution scanning electron microscopy. Oxygen-rich Al-O-N phases were found to be nanocrystalline with a cubic structure like gamma-Al2O3 (spinel), nitrogen-rich phases displayed a hexagonal structure like AlN (wurtzite). Around O/N=1.5 the phase was TEM amorphous.