The early stage phenomena of phase decomposition of some age-hardenable aluminum alloys are demonstrated in terms of the kinetics and microstructures. In particular, the effects of quenched-in excess vacancies and microalloying elements on the early stage clustering behavior are discussed. Based on the pair interactions between atoms and vacancies Monte Carlo simulation was performed. The complex clusters of solute/microalloying element/vacancy were found to be formed and act as nucleation sites for clusters and GP zones. The role of microalloying elements is well understood in terms of the ordering parameters.