The precipitation hardening of an experimental Al-4.2 wt% Mg-0.6 wt% Cu alloy has been studied. After a first initial jump, the yield strength increases almost linearly with the logarithm of the ageing time and a peak of hardness is reached after 11 days at 180 degrees C. Special attention is given to the precipitation hardening during the early stage of ageing. it has been shown that S" phase can be formed heterogeneously on dislocation loops and helices and a new mechanism of precipitation hardening due to this S" phase precipitation is proposed. The precipitation of S" on dislocations is the predominant cause of strengthening during the initial stage of precipitation hardening (up to 30 min at 180 degrees C). Guinier-Preston-Bagaryatsky (GPB) zones (or better, the recently introduced "Cu/Mg clusters") also appear in the initial stage, but their contribution to the hardness. which up to now was considered to be predominant, is shown to be smaller then the one of the S" precipitates. Since the density of the S" nucleation sites is related to the amount of dislocations, this mechanism is important in the case of a bake hardening treatment when ageing is preceded by cold deformation. Uniform S" precipitation has also been found at the later ageing stage, which suggests that the contribution of S" to the precipitation hardening at that stage is not less important. (C) 1998 Acta Metallurgica Inc.