Icosahedral T-2 phases can form either by solid-state precipitation or during solidification in Al-Li-Cu-Mg alloys. The T-2 phase forming during solidification can transform to an R phase at high annealing temperatures. The T-2 phase forming by solid-state precipitation coexists with the Y phase, which has a face-centred cubic (f.c.c.) structure with lattice parameter a approximate to 2.0 nm and can form microtwins with the twin plane of (111). The orientation relationships between the C phase and the T-2 phase are : i ($) over bar 5 parallel to Y[011], Y[113]; i ($) over bar 3 parallel to Y[111], Y[123], Y[115]; Y[235]; i2 parallel to Y[011], Y[111], Y[112], Y[113], Y[115].