Abstract: In this work, microstructure evolution of an Al-3Mg-0.4Cu-0.12Si (wt%) alloy was studied during artificial aging at 180°C and 200°C. The primary investigation methods used were conventional and high resolution transmission electron microscopy (TEM and HRTEM), Z-contrast scanning TEM, and atom probe tomography (APT). Two new metastable phases are found to successively form during 180°C aging, designated as GPB and GPBII zones. The first forming GPB zones are rod-like regions aligned along 100Al directions. As supported by ab initio calculation, the GPB zones are L10 ordered, with alternating Mg enriched and Si, Cu, Al enriched layers. In the later stages of aging, GPBII are detected as nanometer size-scale, homogeneously dispersed, coherent particles. The crystal structure of the GPBII can be explained by a hexagonal lattice, with parameters a=0.7, c=0.4nm and space group P6̄2m. The stoichiometry of the proposed GPBII zone is Al3Mg3Si2Cu1, which in terms of the solute elements agrees with the elemental analysis. Ab initio calculations show very favorable enthalpy of formation of -14.4 kJ/mol. At higher temperature aging and longer aging times, such as 16 hours at 200°C, the microstructure is found to consist of two different crystallographic types of GPBII zones, which can be differentiated based on their location in the matrix. The isolated GPBII zones are comparable to those from lower temperature aging; however, the GPBII zones associated with S-phase particles have a different crystal structure, which seem to be crystallographically related to the structure of lath-like particles in some 6xxx series alloys. At the later stages of aging, the structure also consists of isolated S-phase particles, which have a rod-like morphology and modified orientation relationship (S''-phase). The origin of the modified orientation relationship for S''-phase was studied. It is found that its establishment is related to coherency at the (021)S//(014)Al interface, which is shown to coexist with the commonly present (001)S//(021)Al. Apart from the microstructural studies, attention was paid to the mechanical properties of alloys of similar composition from various aging treatments. The observed strengthening from the early stages of aging was modeled based on a contribution from GPB zones considering three different mechanisms.