A series of Al1-x at % Mgx alloys (x = 3.76, 5.74 and 12.18) has been selected in order to study the mechanisms of the decomposition and precipitation processes which take place as a result of heating of the supersaturated solid solutions. Techniques of differential scanning calorimetry and scanning electron microscopy prodded with an energy dispersive x-ray spectrometer have been used for this purpose. The basis of the Johnson-Mehl-Avrami equation has been applied to characterise the kinetics of the transformations that occur during heating of the supersaturated alloys. Accordingly, an average overall activation energy of 0.51 eV has been determined for the dissolution of Guinier-Preston zones. The kinetics are controlled by the migration of magnesium atoms three- dimensionally in the aluminium matrix. The average activation energies associated with the precipitation of b phase (Mg5Al8) and its redissolution have been determined as 2.42 and 5.695 eV respectively. The reaction order of the precipitation process indicates that the precipitation occurs through the bulk of the alloy in three dimensions and these precipitates grow on pre-precipitated tiny nuclei.