The artificial ageing treatments have been performed in the commercial magnesium alloys AZ80 (Mg-8.2Al-0.5Zn) with different grain size in the temperature interval 150-250 °C. The comprehensive microstructure observations through ageing were carried out by light microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Solid solution decomposition proceeds in non-deformed materials by two modes of precipitated
γ– Mg17Al12phase: discontinuous (grain boundary) and continuous (intragranular) precipitation. A competitive formation of precipitation is influenced by either initial grain size or applied cold deformation before ageing. The electrical resistivity measurements have allowed evaluating some kinetic parameters of precipitated γ- Mg17Al12phase. The good agreements of the obtained volume fractions of γ- Mg17Al12phase after different ageing time by electrical resistivity and a XRD technique were observed.