The constitutions of the Al-rich alloys containing Cu and Mg were investigated by means of thermal analysis and microscopic examination. ation. From various constitutional sections taken as constant in Al-content or taken through the Al axis, the following conclusions were drawn: (1). In a range of Cu 50% and Mg 40%, the primary separations are (Al), CuAl2, S, T, β and X, where S and T being in equilibrium with (Al), denote ternary compounds Al13Cu7Mg8 and Al5CuMg4 respectively, β is an intermediate phase existing between Al and Mg and X is an unknown phase not determined in this investigation. (2). The ternary compounds Al13Cu7Mg8. and Al5CuMg4 were found to exist. (3). The uni-variant reaction lines were found to be: -Liq_??_(Al)+CuAl2, Liq_??_(Al)+S, Liq_??_(Al)+T, Liq_??_(Al)+β, Liq_??_CuAl2+S, Liq+T_??_S, Liq+X_??_S, Liq+X_??_T, Liq_??_T+β, and the in-variant points were; Liq (Al)+CuAl2+S, at 500°, Liq_??_(Al)+T+β, at 447°, Liq+S_??_T+(Al), at 465°, and Liq+X_??_S+T, at 525°. (4). It is concluded that the natural ageing of Duralumin and 24S-type Super-Duralumin is mainly concerned with the solid solubilily change of S-compound and CuAl2 or that of S-compound in Aluminium.