The Al-Cu-Mg-Si alloying system is a base for a diverse group of commercial alloys which acquire their properties after quenching and aging. Therefore, the knowledge of the phase composition of hardening precipitates and the conditions under which they are formed is very important. Vast reference data were analyzed along with experimental results and calculations of phase equilibria. Different alloys were compared based on the composition of the supersaturated solid solution. It is shown that the phase composition of aging products in alloys with Mg: Si > 1 agrees well with the equilibrium phase composition at a temperature of annealing. However, the sequence of precipitation in the alloys with Mg: Si < 1 is more complicated. The hardening in these alloys occurs with precipitation of the β" and θ' phases and their precursors. The former phase may contain copper and later transforms either to β' and β (Mg2Si) or to Q phase depending on the amount of copper and annealing temperature. (C) 2003 Kluwer Academic Publishers.