The analysis of dilatometric and calorimetric curves, completed by optical microscopy observations, permitted to follow the kinetics and the morphology changes of beta'-phase particles during ageing of Al-12 wt.% Mg alloy. It is now well established that beta'-phase particles, which are semi-coherent and metastable, appear initially in a globular shape, transform into thin needles, then into particles of the equilibrium p-phase after a long ageing time, as predicted by the phase diagram. In addition, beta' and beta-phases are formed and dissolved successively and independently one of the other, during non-isothermal ageing. (C) 2003 Elsevier B.V. All rights reserved.