AbstractMetastableδ′precipitates formed at 473 K in Al–1·9Li–2·5Mg alloy have been studied via specific heat measurements, analytical transmission electron microscopy, and atom probe field ion microscopy. The peak temperature of heat absorption associated with the resolution ofδ′into the matrix increases by more than 45 K after prolonged aging, i.e. the thermal stability ofδ′is enhanced by the aging treatment. The coarsening ofδ′obeys Ostwald ripening kinetics. However, the increase in the peak temperature is not related to the increase in size ofδ′but is instead attributed to an increase of the Mg content inδ′precipitates; presumably,δ′changes from Al3Li to (Al, Mg)3Li.MST/1846