Additions of up to 1 wt % scandium have been made to Al, Al-Mg, Al-Mg-Ag and Al-Zn-Mg alloys and the effects on age-hardening and mechanical properties studied. Scandium levels up to 1% could be retained in solution at solidification rates of about 300 Ks(-1). The precipitation of Al3Sc at ageing temperatures in the range 563-593 K (290-320 degrees C) gave significant additional hardening. The low solubility of Sc in the solid state makes it difficult to obtain optimum hardening from Sc and other precipitating elements because of difficulties in solution treatment. The effect of deformation prior to ageing and the temperature-dependent mechanical properties are described.