Using in situ surface X-ray diffraction we have determined the atomic structure and stability of a Pt3Ni(111) surface in the electrochemical environment. Surface segregation leads to a pure Pt(111) skin with enrichment of Ni in the sub-surface atomic layer that determines the surface electronic structure. The Pt-skin surface exhibits inward relaxation upon the adsorption of oxygenated species and this explains the surface stability compared to pure Pt(111). Using Monte Carlo calculations it is shown that nanoparticles with the same surface composition and stochiometry are energetically stable. (C) 2007 Elsevier Ltd. All rights reserved.