Duplex austenite-ferrite stainless steels are susceptible to a variety of decomposition processes when aged within the intermediate range of temperatures (250-500 degrees C). One of these phenomena is the precipitation of the intermetallic G-phase. In the present investigation, the crystal structure and the chemical composition of the G-phase, precipitated in the ferritic phase of an AlSI329 duplex stainless steel, is studied by electron microdiffraction and energy dispersive X-ray spectroscopy. It is determined that the space group of the G-phase is F 4/m (3) over bar 2/m with a lattice parameter four times that of the ferritic matrix. The precipitation mechanism of the G-phase showed a synergetic relation with the ferrite decomposition in Cr-rich and Fe-rich domains. Based on the obtained results, the structural proximity of ferrite matrix and G-phase has been studied. Further analysis allows to suggest that the spinodal decomposition leads to an interdomain of a ferritic structure which is thermodynamically unstable and serves as a precursory site to the development of the G-phase by atomic position readjustments inferior to the atomic distances.