The resistance of pitting of a Fe-17%Cr alloy in Cl- -containing solution (0.5M H2SO4 + 0.3M NaCl) and its relationship with the chemical composition of the passive film have been studied by electrochemical and x-ray photoelectron spectroscopy analyses. Aging of the passive film in the acid solution without or with Cl- (at a potential below the pitting potential) improves the resistance to pitting. The passive films formed in 0.5M H2SO4 (at +500 mV/SHE) have a bilayer structure (hydroxide and oxide) already well developed after 5 min of passivation. The thickness of the passive film (approximately 26 to 28 angstrom) remains constant during polarization. The average cation fraction of chromium in the oxide increases with increasing polarization time (from 35% after 5 min of polarization to 80% after 24 h of polarization). The beneficial effect of aging on the resistance to pitting is attributed to the enhancement of the chromium oxide enrichment at the film/alloy interface. The passive films exposed to Cl- or formed in Cl- -containing solution (0.3 M NaCl) have similar composition and thickness as the films formed in Cl- free solution, but they contain Cl-. The Cl- ions are located mainly in the hydroxide layer (approximately 0.016 mol . cm-3). A very small amount of Cl- is found in the inner oxide layer.