To obtain high-quality produces from aluminium alloys powders, degassing is usually carried out before consolidation. But, in the case of Al-Zn-Mg alloys an evaporation of alloying elements may occur during outgassing. The aim of this work was to determine the influence of degassing conditions (temperature, pressure, nature of the gaseous atmosphere) on the stability of the alloy composition. These studies were performed on X7091 alloy by means of thermogravimetry and temperature-programmed desorption technique. The results show that liberation of alloying elements is closely connected with the evolution of the surface oxide layer which itself depends on various thermodynamic and kinetic parameters. The role of water vapour in the gaseous environment was particularly emphasized.