The compositions of passive films formed on Fe-17Cr-13Ni (at. %) and Fe-18.5Cr-14Ni-1.5Mo (100) single crystals have been determined and the structure of the alloy under the film has been investigated. The alloys were passivated in 0.05M H 2SO 4 at 250 mV/SHE for 30 min. The oxygen content was measured by nuclear microanalysis using the 160(d,p) 170 ∗ reaction. The oxygen content in the passive film is similar for the two alloys and equal to (12±2) 10 15 0/cm 2. The cationic compositions of the passive films have been determined by 4He channeling at two incident beam energies: 0.8 and 2.0 MeV. For the two studied alloys, a total cation content of (5±2)10 15 at/cm 2 is found in the passive films. The corresponding thickness is about 12 Å. There is an excess of oxygen, which can be attributed to the presence of hydroxyls and sulfate. A strong chromium enrichment is found in the passive film formed on both alloys: chromium represents about 50% of the cations. There is no evidence of molybdenum enrichment in the passive film formed on the Mo-alloyed stainless steel. The comparison of the results obtained at the two different incident beam energies (0.8MeV and 2MeV) reveals the existence of defects at the alloy/passive film interface.