For a better understanding of the physical and chemical processes underlying the deposition of aluminium nitride and AIN, thin films by reactive magnetron sputtering of aluminium, we compare sputter gas mixtures of argon and nitrogen with mixtures of argon and oxygen. Depending on the flow rates of the reactive gases, their partial pressures are detected by mass spectroscopy and the aluminium densities in the plasma ring are observed by optical emission spectroscopy. The addition of oxygen results in two stable modes, the reactive mode, corresponding to a target surface almost covered by reaction products, and the metallic mode, corresponding to a metallic target surface. Avalanche-like transitions between these two modes lead to hysteresis loops. On the contrary, using nitrogen, the discharges are in a stable state also for partially covered targets. In this case, there is a gradual transition from a metallic target, which is only found at pure argon, to a maximum-covered target. Only small hysteresis effects and no avalanche-like transitions are observed. We conclude that, if sputtering aluminium targets, the addition of oxygen or nitrogen as the reactive gas to argon discharges represents two different model cases of the reactive sputtering process.