Samples of the commercial pressure-diecast alloy ZA8 containing 8.1 % Al, 1.1 % Cu and 0.024% Mg have been examined by transmission electron microscopy (TEM). During decomposition of the high temperature f c c-beta phase, on cooling after casting, a transitional phase was formed within the alpha-phase in both dendrites and eutectic. This phase was identified as the metastable phase alpha'(m) containing 11% Al or 23% Al, with a f c c crystal structure and lattice parameter (at 11%) of about 0.394 nm. It had adopted a symmetrical cube/cube orientation relationship with the surrounding f c c alpha-phase. The stability of this metastable phase at ambient temperatures was greatly increased by the presence of copper.