Beneficial effects of underageing in enhancing the creep resistance of certain aluminium alloys may be diminished or lost if the alloys are held at close to ambient temperatures prior to testing. This problem is associated with undesirable secondary precipitation during the dwell period and may be overcome by slow cooling the alloys from the initial ageing temperature. A method for successfully exploiting secondary precipitation to improve the mechanical properties of aluminium alloys has been developed which involves interrupting artificial ageing with a low-temperature dwell period. Average increases in 0.2% proof stress of 10-15%, combined with improved fracture toughness, have been achieved in a wide range of alloys when compared with equivalent properties obtained by conventional T6 tempers. These effects arise because interrupted ageing promotes nucleation of more finely dispersed precipitates in the final microstructures.