We report on an X-ray photoemission spectroscopy investigation of the early stages of growth of ultra-thin Cr films on the oxygen-passivated Fe(0 0 1)-p(1 x 1)O surface. The Cr coverages ranged from sub-monolayer up to a few atomic layers. Cr has been grown either at 380 K or at 570 K. Our investigation reveals that during the Cr film growth oxygen floats toward the free surface. The presence of a metallic Cr signal from the very beginning of film growth is discussed in relation to Cr-Fe intermixing and alloy formation at the interface. Our findings are independent from the growth temperature, indicating that it has a very little influence on the chemical interactions at the interface, at variance with the oxygen-free Cr/Fe interface. (C) 2012 Elsevier B. V. All rights reserved.