The precipitation and the strength evolution during equal channel angular pressing performed at 180 A degrees C in an Al-4.2wt% Mg-1.2wt%Cu alloy have been studied by room temperature compression tests and transmission electron microscopy. The age hardening behaviour of these AlMgCu alloys, in which the precipitation sequence involves the S-phase and its precursors, was investigated and revealed a yield strength peak after 8 days at 180 A degrees C. The influence of the Severe Plastic Deformation on the microstructure and mechanical properties of under-aged and peak-aged samples are presented. Notably, in the under-aged sample, a gradual increase of the strength after each ECAP pass is obtained while, the peak-aged samples loose much of their strength during the first ECAP pass. TEM characterization of the microstructure before and after ECAP is presented and linked to the evolution of the mechanical properties.