Two age-hardenable AlCuMg alloys (Cu 4.5wt% and Cu/Mg=2 and 8) have been studied by DSC, electrical resistivity and microhardness, joined by TEM in order to investigate the rapid hardness increase taking place in the early stages of decomposition of a supersaturated solid solution after quenching. Exothermic DSC signals on as-quenched samples, double-peaked isothermal resistivity traces, microhardness measurements and TEM/SADP observations suggest that the rapid hardness increase should be attributed to GP/GPB zone formation followed by a rearrangement of the zones into intermediate structures S"/theta".