Clustering and precipitation reactions during the early stages of ageing at 180°C in an Al-1.7Cu-0.3Mg (at.%) alloy and equivalent alloys containing 0.1 and 0.2 at.% Ag have been studied. Transmission electron microscopy, electron diffraction and atom probe field ion microscopy have been used to clarify the precipitation processes in these alloys following ageing for 15, 30, 120, 720 and 9000 s. Reactions involving the formation of Mg and Mg-Cu co-clusters were observed in the ternary Al-Cu-Mg alloy whereas Mg, Ag and Mg-Ag co-clusters were found to co-exist with Cu-clusters in the quaternary Al-Cu-Mg-Ag alloy. Results indicate that the Mg Ag interaction is strong and it is proposed that the role of Ag in quaternary alloys is to effectively trap Mg atoms, resulting in Mg-Ag co-clusters, which provide favorable sites for nucleation of the Ω phase. Various models for precursor phases have also been examined. Apart from the presence of Mg-Ag co-clusters, no evidence of a distinct precursor phase was observed.