X-ray photoelectron spectroscopy (XPS), a powerful method for investigating composition and thickness of thin films in the nanometer range, has been used to characterize the surface of two stainless steels with different bulk composition, DIN 1.4301 (18%Cr 8%Ni) and DIN 1.4529 (20%Cr 25%Ni 6%Mo) after mechanical polishing and after potentiostatic passivation in a wide range of potentials in sulfate solutions at different pHs. The XPS results confirm that the passive film thickness increases with applied potential, its composition is enriched in chromium oxy-hydroxide after immersion in acidic solutions and polarization at low potentials whereas oxidized nickel is depleted. Special emphasis is given in this paper to the composition of the metal phase underneath the passive film. It is shown that the composition of the interface is strongly enriched in nickel for both stainless steels being 28 ± 2 wt% Ni versus a value of 8% in the bulk and 45 ± 2 wt% versus 25 wt%Ni in the bulk, respectively. The composition of the interface for a given alloy remains unchanged in a wide range of conditions of passive film formation such as pH, applied potential, passivation time. These results are discussed with respect to the influence of the interface composition on the corrosion rate of alloys in the passive state and on their pitting resistance.