Al-rich Al-Cu-Mg alloys have wide range of applications for automotive and aerospace where the control of the original solidification microstructure is important to achieve desired properties. Under constant temperature gradient G (4.84 +/- 0.13 K.mm(-1)), a series of directional solidification experiments were performed at 6 different growth rates V (16.7 - 166.7 mu m/s) and 6 different Mg contents C(0-Mg) (0 - 5 wt.% Mg) of Al - 5wt.% Cu - (0-5)wt.% Mg alloys. The solid-liquid interface was investigated in as-quenched specimens. Primary dendrite arm spacing XI was measured on longitudinal and transverse sections of the specimens. The variations of lambda(1) with respect to V and C(0-Mg) were determined and the results were compared to the related theoretical models and the published data.