The influence of the addition of copper on the precipitation behaviour of an Al-Zn-Mg alloy at 160°C has been investigated. Phase transformation has been investigated by differential scanning calorimetry, the precipitation kinetics by small angle X-ray scattering and transmission electron microscopy, and the mechanical properties by microhardness testing. The addition of copper results in an increase in the stability of Guinier-Preston zones formed at room temperature, and in a decrease in the micleation temperature for the η′ phase. It also results in a decrease in the apparent diffusion coefficient for precipitation, and in an increase in the strengthening ability of precipitates. As a consequence, the copper bearing alloy shows a much higher maximum strengthening ability, and a lower sensitivity to the heating rate to the aging temperature. However, the higher super saturation of the copper bearing alloy results in a higher quench sensitivity, and at slow quench rates the ternary alloy shows a higher strengthening potential. The experimental data are quantitatively compared with the results of a precipitation model, describing the evolution of precipitate size, volume fraction, and yield stress, and values for physical parameters of the precipitation and strengthening mechanisms are obtained.