We report on first-principles calculations of the thermodynamic stability of the passive film on FeCr alloys. Using density functional theory we calculate the total energy of a sequence of spinels Fe(3-2x)Cr(2x)0(4) (0.0 <= x <= 1.0) relative to magnetite (x = 0.0). We use the calculated energy of formation to deduce Pourbaix diagrams for the alloy/oxide system as a function of oxide Cr content and show that these are consistent with experiments if one takes into account the observed Cr enrichment of these films. Indeed, these calculations show that the observed enrichment is a necessary requirement for passivity. (C) 2007 The Electrochemical Society.