The effect of an Mg addition on the work hardening of high purity (4N) aluminum was investigated over a wide range of strain. The analysis of the true stress-true strain (sigma - epsilon) curves of samples after annealing and equal-channel angular pressing (ECAP) shows that the addition of 3wt% Mg strongly influences the rate of work hardening of Al. Investigations using both mechanical testing and transmission electron microscopy (TEM) reveal the decreasing effect of Mg on the recovery properties of Al from the very early stage up to at least epsilon = 8 of plastic deformation. It is shown that the sigma - epsilon relationship can be accurately described by an exponential power law constitutive relationship over a wide range of strain for pure Al but for the Al-Mg alloy this constitutive equation cannot be extended to high strains.