In the present study the oxidation state and the sui face composition of a ternary PtRuCo catalyst were investigated under methanol steam reforming (SRM) and water gas shift (WGS) reaction conditions at 570 K Ambient pressure X-ray photoelectron spectroscopy (APPES) was applied in situ at 05 mbar while simultaneously monitoring the catalytic activity of the sample by on-line mass spectrometry Non-destructive depth profile measurements performed under SRM reaction conditions over a polycrystalline PtCo foil were also used to obtain detailed depth-resolved information The results showed that surface segregation of cobalt and modification of its oxidation state occurs when switching from SRM to WGS reaction conditions Evidence of ionic Pt was found only during WGS reaction while Ru was mainly present in the metallic state The results clearly demonstrate the dynamic response of the PtRuCo catalytic surface to the reaction atmosphere (C) 2010 Elsevier B V All rights reserved