The ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenic duplex stainless steel undergoes a variety of decomposition processes when aged in the temperature range 650 - 750degreesC. These processes involve the precipitation of the austenite and of the sigma and chi Frank-Kasper phases. The intermetallic chi-phase is found at both the grains boundaries ( homo and heterophase interfaces) and inside the ferritic grains where it adopts an unexpected hexagonal shape. At the early stage of its precipitation, it nucleates at the delta/gamma and delta/sigma heterophase interfaces and then grows by expanding exclusively in the ferritic matrix. This study is basically focused on this intermetallic chi-phase. The crystal structure and the chemical composition are respectively studied by electron diffraction and energy dispersive X-ray spectroscopy. The chi-phase exhibits rational orientation relationships with the austenite and the sigma-phase with which it is in contact and an invariably cube-on-cube orientation relationship with the ferritic matrix into which it grows. Based on the orientation relationship, the morphology and the number of variants of this.-phase are understood in terms of the group theory. The planar defects present in a large density in the chi- phase, are roughly parallel to {011}(chi)//{011}(delta). The fault vectors are determined as: 1/3 [110](chi) and 1/4 [111](chi), the latter corresponding for a bcc structure to a pi phase shift, the defects can be simply described as pi boundaries. Based on the obtained results, a structural proximity between the chi-phase and a super-cell derived from the ferritic matrix has been brought to light. This super-cell is described as a stacking of corrugated and planar layers obeying the following parallelism {011}(chi)//{011}(delta). Indeed this super-cell approach provides an interpretation for several microstructural features such as the chi/delta interface plane, the planar defects in the chi-phase and their related fault vectors.It has been also stated that Mo is an efficient chi-phase forming element. Upon these considerations a detailed characterization provided valuable insights into the precipitation mechanism associated with the chi-phase formation. (C) 2004 Kluwer Academic Publishers.