The effect of Mg content on the natural ageing of Al-Cu-Li-(Mg) alloys has been investigated on a compositionally graded material made by linear friction welding, allowing to probe Mg contents between 0 and 0.4 at.%. High throughput time-and space-resolved characterization of natural ageing kinetics has been achieved using small-angle X-ray scattering, supplemented by differential scanning calorimetry. Natural ageing results mainly in the formation of Cu-rich clusters whose characteristics strongly depend on the presence of Mg. Larger precipitates of 2 nm size also form to a lesser extent above a Mg concentration of 0.1 at.%.