Discontinuous precipitation and dissolution in the Al-Zn alloy system has been the subject of many theoretical and experimental investigations that have contributed to the understanding of the different mechanisms which control them. However, many questions remain unanswered because of the complexity of the constituted phases which are affected by the rate of quenching, the amount of deformation, the temperature of homogenization and the ageing effect The purpose of this work is to clarify the effect of temperature and deformation on the mechanisms of these two reactions during ageing of the AI-15at.%Zn alloy. The techniques of analysis used in this respect are optical microscopy, X-ray diffraction and Vickers microbardness.