Atom probe field ion microscopy (APFIM), transmission electron microscopy (TEM), and microbeam electron diffraction have been used to study the early stages of precipitation in an Al-1.1Cu-1.7Mg (at%) alloy. This alloy exhibits a large and rapid hardening reaction upon ageing for as short as 60 s at temperatures higher than 130 °C. Results from APFIM suggest that the formation of Cu-Mg co-clusters, not resolved in TEM, is responsible for the rapid hardening reaction. A period of constant hardness follows the rapid initial reaction, during which the clusters develop into ultra fine rod-like precipitates, considered to be GPB zones and precipitation of the S phase (Al 2 CuMg) occurs heterogeneously on dislocations. Like the initial clusters, the GPB zones were found to be rich in Cu and Mg. Peak hardness was associated with the further dispersion of S precipitates in the matrix and further growth of GPB zones, suggesting the following precipitation sequence: solid solution → pre-precipitate stage → GPB zones + S → S.