A set of extrusion samples of Al-Si-Mg alloys (0.5 wt%Mg and 0.3-0.8 wt%Si) were, respectively, T1- and T4-heat-treated. Differential scanning calorimetry (DSC) was used to heat the samples to particular temperatures to promote the formation of precipitates for study by transmission electron microscopy (TEM). It was found that, apart from beta ", beta', and B', there were many precipitates showing rectangular lattices when viewed along the long axes of the precipitates. It is considered that the residual stresses (or dislocations) in the extrusion after the TI-treatment facilitated the nucleation and growth of the precipitates during the heating in the DSC. (C) 2001 Elsevier Science Ltd. All rights reserved.