Zirconia can be stabilized by incorporation of nitrogen. The phase relationships in the ZrO2-rich part of the system ZrO2-Zr3N4 have been investigated using high-temperature X-ray methods. At temperatures above 1000 degrees C, a tetragonal and a cubic phase with randomly distributed vacancies exists, depending on the amount of incorporated nitrogen. This high-temperature behaviour is similar to that of systems like ZrO2-Y2O3, which is another indication for the important role of anion vacancies in zirconia systems. Below 1000 degrees C, beta-type phases with an ordered arrangement of anion vacancies are stable.