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Precipitation behaviour of Al–Mg–Si alloys, with balanced (Mg/Si = 2), excess silicon (Mg/Si < 2) and excess magnesium (Mg/Si > 2) compositions, were studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and Vickers hardness tests. Four significant exothermic peaks were observed in DSC curves which were attributed to metastable clusters, β″, β′ and stable β phases. The peaks corresponding to β″ and β′ were formed closely in the DSC curves but showed different behaviour in isothermal annealing. The additional peak verifying the precipitation of phases, which has recently been proposed by some workers, was not detected. Transmission electron microscope observations and Vickers microhardness tests showed that β″ precipitates played a major role in improving the hardness, but not β′ precipitates.