Transmission electron microscopy (TEM) and microhardness indentation techniques have been used to examine the microstructural evolution and age hardening behaviour of two Al-Cu-Mg alloys containing 1.1 atomic percent Cu and up to 0.5 atomic percent Mg. Carefully controlled ageing treatments followed the schedule of: solution treatment (525degreesC) + water quench (23degreesC) + (a) artificial ageing (150degreesC) or (b) natural ageing (up to 24 h at 23degreesC) + artificial ageing (150degreesC). The implementation of a natural ageing step at 23degreesC following quenching significantly influences the characteristics of precipitate nucleation and growth, resulting in (1) a more uniform precipitate distribution and (2) the clearly increased incidence of an additional precipitate phase (possibly Omega) residing parallel to {111}(alpha) matrix planes and in association with former dislocation loops.