Numerous efforts have already been made to optimize nitridation of crystalline sapphire (c-Al₂O₃) substrates whereas very little attention has been paid to nitridation of amorphous aluminum oxide layers (a-AlO_x). An extensive analysis of the reaction of amorphous aluminum oxide films with nitrogen species is thus needed to clarify the mechanisms of nitrogen incorporation into such layers and to control their properties. In this work X-ray photoelectron spectroscopy was used to determine the chemical state of nitrogen formed by nitrogen plasma treatment of c-Al₂O₃ and 15 nm thick a-AlO_x layers grown by atomic layer deposition on Si and sapphire substrates. The results show that the nitridation proceeds significantly different for c-Al₂O₃ and a-AlO_x samples, which we correlate with the initial stoichiometry of both materials. At the surface of sapphire O vacancies were found, which are necessary for the formation of AlN-type bonding via diffusion limited replacement of oxygen by nitrogen. This process was slow and involved formation of oxinitride AlN–O. After 80 min of nitridation only ∼3.4 at% of N was incorporated. In contrast, in a-AlO_x layers Al vacancies were present before nitridation. This opened a new, more effective path for nitrogen incorporation via accumulation of N in the cation-deficient lattice and creation of the Al(NO_y)_x phase, followed by AlN and AlN–O formation. This scenario predicts more effective nitrogen incorporation into a-AlO_x than c-Al₂O₃, as indeed observed. It also explains our finding that more N was incorporated into a-AlO_x on Si than on sapphire due to supply of oxygen from the sapphire substrate.