A physico-mathematical model has been developed, based on the Point Defect Model, to simulate depth profiles of the alloying and host elements in the passive systems (including barrier and outer films and the metal phase) of binary alloys. Curve fitting of the XPS depth profiles for W and Ni in a Ni-W alloy shows that the Point Defect Model can account for the segregation of alloying elements in the passive film on this system. The effects of various parameters on the segregation have been investigated. Most of the predictions of the model are experimentally observed.