The evolution of vacancy-solute complex in Al-Mg based alloys with different Mg contents during aging process was studied by positron annihilation spectroscopy together with Vickers micro-hardness and transmission electron microscopy measurements. For quenched Al-Mg based alloys, no obvious change in positron lifetime is observed during natural aging process. While during artificial aging at 180 degrees C after subsequent quenching, the positron lifetime and Doppler broadening S parameter show a fast decrease after the initial 1 min aging. Further aging for more than 10 min causes increase of these parameters. Coincidence Doppler broadening measurement indicates formation of vacancy-Mg complexes even in the as-quenched samples. Natural aging at room temperature has no effect on these vacancy-solute complexes. It is believed that with artificial aging the microstructure of Mg transforms from uniformly scattered vacancy-Mg complexes to Mg clusters. The Vickers micro-hardness changes slightly during the artificial aging, which is supposed to be related with the formation of Mg clusters. (C) 2014 Elsevier B.V. All rights reserved.