The precipitation processes during elevated temperature ageing of Al-4Cu-(Mg, Cd) (wt. %) alloys have been studied using transmission electron microscopy and three dimensional atom probe (3DAP). Enhanced precipitation of theta' (Al2Cu) was confirmed in Cd-containing alloys. Additions of Cd into the Al-Cu-Mg alloys also stimulated the precipitation of the sigma phase (Al5Cu6Mg2). In the ternary Al-Cu-Cd alloy, elemental Cd particles were detected in a uniform dispersion throughout the matrix and were attached to theta', while in the Al-Cu-Mg-Cd alloy, co-clustering of Cd-Mg was observed at early stages of ageing. This result suggests that the enhanced precipitation and associated hardening in the quaternary Al-Cu-Mg-Cd alloy is initiated by the Cd-Mg co-clusters, through what is called as cluster-assisted nucleation. The nucleation mechanism in the ternary Al-Cu-Cd alloy is almost certainly the same, although the chemistry of the initiating cluster which assists nucleation is different, which is thought to be Cd clusters.