Pd-Cu alloys have been investigated as promising candidates for hydrogen separation membranes. Surface segregation influences the long-term performance of these membranes since their catalytic effect is mainly controlled by the surface composition. In the present research, surface segregation of Pd-40 at.% Cu alloy in vacuum and various gas atmospheres (H-2, CO and CO2) was investigated with both XPS and LEISS probing different depths below the surface. Adsorption of H-2 and CO on the surface has a significant impact and the surface segregation trend can be reversed as compared to segregation in vacuum, however, CO2 has almost no influence on the segregation behaviour. A thermodynamic model is also presented to explain these phenomena and to understand surface segregation behaviour of binary alloys in various gas atmospheres. The results can be considered as basic guidelines to design novel alloys for hydrogen separation membranes and predict their long-term performance under actual working conditions. (C) 2020 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.