A systematic study of the precipitation processes in high strength Al-Zn-Mg-Cu alloys has been conducted. A series of experimental alloys was prepared with varying Zn:Mg ratio, but equivalent total solute content, to be representative of those used in applications which demand a combination of strength, fracture toughness and resistance to environmentally sensitive cracking mechanisms. Artificial ageing curves were constructed for each alloy, based upon 7xxx series duplex treatments widely used in industry. Ageing kinetics were investigated beyond peak strength into the overaged condition, since this is known to promote the most favourable balance of properties. Differential scanning calorimetry (DSC) and transmission election microscopy (TEM) observations were made on the alloys throughout the ageing process to fully understand the precipitation events occuring. For a particular overageing treatment, higher Zn:Mg ratio alloys were consistently found to be at a more advanced stage of precipitation while higher strengths were retained at low Zn:Mg ratios. Grain boundary characteristics, such as particle size and precipitate free zone width, were also influenced by Zn:Mg ratio at a given strength level. This paper provides new understanding of precipitation kinetics and the control of important microstructural features which influence the balance of properties in 7xxx series aluminium alloys.