AbstractA combination of transmission electron microscopy, atom probe tomography and high-energy X-ray diffraction was employed to investigate the influence of local microstructural changes on strengthening in a commercial Al-Li-Cu based alloy, AA2198, in the stretched and naturally aged, and overaged states. Strengthening in the stretched and naturally aged temper was shown to be governed by a combination of Cu-Cu clusters, δ′/β′ phase and solution strengthening. This is in contrast to another report which suggest that strength in this temper is only due to Cu-rich clusters [Decreus B et. al. Acta Mater 61 (2013) 2207]. On the other hand, although large volume fractions of equilibrium phases such as TB, and θ were present in the overaged temper, its strengthening was largely governed by order hardening, which is the strengthening mechanism associated with the δ′/β′ phase. The δ′/β′ phase remained in the matrix even after extensive overaging.