The aging processes in Al-Mg-Si alloys have been studied by using differential scanning calorimetry (DSC), microhardness measurements (HV) and X-ray diffraction (XRD). Five processes have been detected by the DSC curves and HV behaviour. In the DSC curves, four exothermic and one endothermic reactions are developed. The sequence of processes are Guinier-Preston (G.P.) zones, dissolution of the G.P. zones, intermediate precipitation of W'-phase, precipitation of W -phase and precipitation of the stable b'-phase + Si particles. The activation energies associated with the processes have been determined by using Kissinger method. Consequently, the nucleation mechanism of the precipitates can be explained. These phases are confirmed by XRD analysis.