High strength AlZnMgCu alloys of greater magnesium content than usually in 7000 series were examined. An analysis of multi-stage heat treatments involving retrogression and re-ageing (RRA) was performed. The phase transformations proceeding during subsequent steps of that complex heat treatment were studied using differential thermal analysis (DTA) and electrical resistance measurements. Based on the behaviour of dissolution processes of particles strengthening the solid solution, the optimum parameters of RRA were determined and good mechanical properties were achieved. The measurements of electrical resistance at ambient temperature enabled establishing optimum conditions of retrogression (temperature and time), affecting the mechanical properties and specific electrical resistivity, which are considered to be a good measure of the alloy's susceptibility to stress corrosion cracking.