Formation of meta-stable surface-alloys can be used as a way to tune the binding strength of reaction intermediates and could therefore be used as improved catalyst materials for heterogeneous catalysis. Understanding the role of adsorbates on such alloy surfaces can provide new insights for engineering of more active or selective catalyst materials. Dynamical surface changes on alloy surfaces due to the adsorption of reactants in high gas pressures are challenging to investigate using standard characterization tools. Here we apply synchrotron illuminated near ambient pressure X-ray photoelectron spectroscopy to probe the surface structure of a CuPt model system. We report an investigation of the formation of a CuPt surface alloy induced and stabilized by exposure to mbar pressures of CO. The location of Pt within the alloy is assessed by deconvolution of the Pt 4f core level spectra into surface Pt and bulk Pt contributions. The study provides direct evidence on how it is possible to monitor the surface structure under near operation conditions. (C) 2014 Elsevier B.V. All rights reserved.