This study investigated the effect of aging on the precipitation and kinetics of dissolution of second phase Mg₁₇Al₁₂ in AZ91 magnesium alloy (Mg- 9 wt. % Al- 1 wt. % Zn), using X-ray diffraction, microhardness measurements and differential scanning calorimetric analysis (DSC). With the last instrument, the all samples were heated from room temperature to 400 °C, at heating rates of 10 to 30 °C/min.  The results were supplemented by measuring the average of activation energies, using isothermal treatments by Johnson−Mehl−Avrami (JMA) methods and by non-isothermal treatments using Ozawa, Boswell, Kissinger, Augis and Bennett and Mahadevan methods were around 129,694 and 116.781 kJ/mol. the numerical factor m and the Avrami parameter n is estimated to be approximately equal to 1.211 and 1.152 respectively. This value corresponding that the bulk nucleation with varying number of nuclei was dominant in one-dimensional (needles) controlled by interface reaction.