The present reference steels for the Phenix fuel subassembly are 15-15T1 for the clad and EM 10 for the wrapper. In this paper the behavior of these materials is described by analyzing the main macroscopic properties (swelling, irradiation creep, tensile, Charpy) and the microstructural evolution observed after irradiation in Phenix up to 90 dpa NRT. The first part deals with the 15-15Ti behavior. After having given the available nondestructive (profilometries) and destructive (density measurements) results and described the main parameters acting on the dimensional stability of the Phenix cladding, TEM examinations performed on neutron-irradicated clad samples are presented. No recrystallization occurred either in the upper part of the pins or at the maximum deformation level. The highly swelling-resistant clad exhibited fine and homogeneous distribution of gamma prime phase. The tension tests performed on these pins showed that the mechanical behavior of 15-15Ti not only depended on the test and irradiation conditions but also on the swelling resistance. Concerning the wrapper material, we first show the excellent dimensional stability of EM 10 and Fl7 irradiated as Phenix wrappers up to 90 dpa NRT. The profilometries exhibited significant deformation, only in the lower part of the hexagonal cans where the maximum void swelling is lower than 1%. The mechanical tests performed on specimens irradiated in an experimental capsule or machined from irradiated wrappers showed that the mechanical properties of these materials are modified by irradiation only for conditions appropriate to the bottom of the wrapper. Basically, a tensile hardening and a loss of ductility with an increase in Charpy embrittlement is observed. This tendency was evident for the ferritic Fl 7 and for the ferritic-martensitic EM 12, but for the unstabilized martensitic EM 10 the irradiation-induced degradation of the mechanical properties was weak. Finally, these results are discussed using TEM observations that demonstrated the excellent microstructural stability under neutron irradiation of the EM 10.