The precipitation state of the Al-Cu-Mg-Si and Al-Mg-Si alloys was investigated by TEM, EDS and HREM. In the Al-Cu-Mg-Si alloys, it has been shown that a structural phase transition exists between 3 phases: Q-Al5Cu2Mg8Si6, QC and QP. These phases, which have close compositions, are based on the same lattice, the QP lattice. A model of this transition has been developed by considering the ordering on the QP latent lattice of triangular clusters (found from the Q structure determined by X-ray). The model has been applied to the β′ and B′ phases in the Al-Mg-Si alloys. It has permitted to determine the structures of QC and β′, precise the mechanisms of ordering and perform Monte Carlo simulations on a honeycomb lattice. © 2000 Trans Tech Publications.