The precipitation behaviour of three Al-Mg-Si alloys has been studied by differential scanning calorimetry (DSC) and transmission electron microscopy. Seven exothermic peaks are usually observed in DSC thermograms of Al-Mg-Si alloys with high silicon contents. These peaks are believed to be caused by the formation of clusters of silicon and magnesium atoms, Guinier-Preston zones, small precipitates with an unknown structure, beta" phase, B’ phase, silicon precipitates and beta phase. The silicon content has no obvious effect on the precipitation of beta" and B’ phases in these alloys, but it has a pronounced influence on the formation of silicon precipitates. The cold-rolling reduction before solution heat treatment is also found to affect the precipitation process appreciably.