The effect of c-plane sapphire nitridation upon exposure to a rf N 2 plasma at temperatures in the range 100-700 °C on the quality of GaN epilayers grown by MBE is investigated. A homogeneous AlN layer is formed at 200°C. Nitridation at higher temperatures causes rough and non-homogeneous nitridated layer including both AlN and NO. Lowering the nitridation temperature to 200 °C results in the improvement of the GaN structural and photoluminescence properties. The results are interpreted in the framework of a chemical model based on the competition between formation of AlN and NO whose adsorption/desorption equilibrium on the sapphire surface strongly depends on temperature.