In this study, atom probe tomography was used to investigate the microstructure of the alloy AA2198 (Al–1.35 Cu–3.55 Li–0.29 Mg–0.08 Ag) over a range of ageing conditions to examine the evolution of phases in the alloy, in particular aiming to reveal the nucleation mechanism of the strengthening T1 phase. T1 precursor phases were observed from early ageing, most of which were connected to dislocations enriched with Mg and Ag. This Mg solute segregation on the dislocations was subsequently observed to develop into S-like phases. Ag and Mg segregation to T1 interfaces was systematically observed when the plates were oriented perpendicular to the probing direction. The matrix solute content was followed during the course of T1 precipitation. It was found that the evolution of Cu, Li, Mg and Ag was similar, giving additional evidence for their co-precipitation.