Resistivity and electron microscope measurements have been used to study the effects of deformation on the characteristics of ′ precipitation in an Al 5·12% Cu 0·14% Cd alloy. The deformation was applied both before and simultaneously with ageing. Material subjected to large amounts of deformation (10–60%) before ageing at 180°C, subsequently produced ′ precipitation which obeyed a (time)⅔law. A kinetic analysis gave results which were consistent with ′ precipitation on dislocations, and electron microscopy confirmed this interpretation.