A study of the effect of magnesium concentration on the ageing behaviour as measured by the hardness of 380 alloy was conducted for three levels of magnesium, namely 0.06 (base alloy), 0.33 and 0.5 wt%, for water-chilled castings (dendrite arm spacing similar to 10-15 mu m). Differential scanning calorimetry analysis of as-cast samples was carried out to determine the changes in the reactions of the phases obtained during alloy solidification, employing heating rates of 0.1 and 1.0 degrees C s(-1), up to approximately 700 degrees C. Two heat treatments were applied to the as-cast alloys : T5 comprising ageing at 25 (room temperature), 155, 180, 200 and 220 degrees C, for times up to 200 h, and T6 comprising solution heat treatment at 480 degrees C or 515 degrees C for 8 h, followed by quenching in warm water at 60 degrees C, followed by immediate artificial ageing at 155 or 180 degrees C for varying times up to 100 h. The results show that the higher hardness values obtained with T6 treatment can be explained by the excess precipitation of magnesium-containing phases in the as-solidified alloys. This precipitation could be eliminated under the high cooling-rate conditions prevalent in die-casting operations so that T5 treatment may be used to replace T6 treatment to produce the same hardness values. In addition, solution heat treatment in the low-temperature range (480-515 degrees C) is adequate to produce the required changes in silicon morphology and dissolution of magnesium in the matrix. No significant difference in hardness behaviour was observed when the magnesium content was increased beyond 0.3 wt %.