Weldalite 049 is a weldable, high strength Al-Cu-Li based alloy with trace amount of Mg and Ag [1, 2]. Major phases observed in the peak hardness condition are spherical, semicoherent {001} plate of and coherent {111} plate of T1 phase [3, 4]. Langan and Pickens [5] attributed the predominant strengthening phase in Weldalite 049 to the T1 phase that precipitate uniformly in the matrix. While T1 phase is known to precipitate heterogeneously on dislocations and grain boundaries in Al-Cu-Li based 2090 alloys [6], addition of Mg and Ag leads uniform dispersion of the T1 plates in the matrix [7, 8]. Because Weldalite-type alloys show the highest strength of wrought aluminum alloys, the mechanism by which the T1 plates are uniformly dispersed has been a subject of many investigations [7–10]. The role of trace additions of Mg and Ag on the nucleation of plates on the {111} planes in Al-Cu-Mg-Ag alloy has been studied by many researchers [12–16]. In an Al-Cu-Mg-Ag quaternary alloy, plate-like phase precipitates on the {111} matrix planes in addition to the phase on the {001} planes, which causes enhanced tensile strength and creep properties [12, 13]. We have recently investigated the evolution of plate-like precipitates of the phase in Al-Cu-Mg-Ag alloy by three dimensional atom probe (3DAP)[15, 16], and found that Ag and Mg atoms form co-clusters after several seconds of artificial aging, and that these evolves to plates continuously as aging goes on. In addition, strong segregation of Ag and Mg atoms have been found on the broad interface of/interfaces within a few atomic layers, which is believe to relieve the coherent strain at the/interface. Trace additions of Ag and Mg to Al-Li-Cu alloy similarly causes a uniform dispersion of the plate-like T1 phase [7–10]. However, the mechanism how Ag and Mg promote the nucleation of the T1 plates in Al-Li-Cu alloys is still not clear, although a mechanism similar to that for an Al-Cu-Mg-Ag alloy is speculated. Itoh et al.[8] reported three types of heterogeneous nucleation sites for the T1 plates, ie, dislocation loops, octahedral voids and Mg enriched {111} GP zones, and they reported that the T1 nucleated at the octahedral void does not contain Mg and Ag, while the one nucleated at {111} GP zones contain Mg and Ag atoms. Although Ag and Mg atoms appear to be incorporated in the T1 phase, their location, ie whether or not they are segregated to the T1/interface, has not been successfully determined because of the thinness of the T1 plate (one unit cell). In this study, early aging stage of Weldalite 049 alloy has been examined by 3DAP in order to clarify the role of Ag and Mg atoms for the uniform dispersion of plate-like {111} precipitates. In addition, how Ag and Mg atoms are incorporated in the T1 plate has been investigated by characterizing the local chemical composition of the T1 plates.