The microstructural evolution of an Al–Zn–Mg–Cu alloy under continuous heating over the temperature range of 298 to 648 K (25 to 375 °C) is characterized by focused ion beam-scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The chemical composition, dimensions, number density, and volume fraction of precipitates are measured comprehensively. Quantitative measurements of Guinier-Preston zones volume fraction and η′/η phase volume fraction are correlated to the viscoelastic response of the Al–Zn–Mg–Cu alloy with temperature in the studied range. This provides valuable evidence indicating that observed variations in the viscoelastic behavior of the Al–Zn–Mg–Cu alloy may be associated with phase transformations involving Guinier-Preston zones and phases η′ and η.