Using density functional theory (B3LYP functional) we systematically studied Pt/Ni alloy clusters ( less than or equal to 44 atoms) and found that the Pt/Ni(111) alloy surface prefers the bulk structure (Pt3Ni crystal), without showing surface segregation. This leads to an ordered surface with each Ni surrounded by 6 Pt atoms. We then examined the oxygen chemisorption at all surface sites and found strongest binding at a FCC site (2 Pt and 1 Ni atom, 3.50 eV). On pure Pt(111) oxygen is most stable (3.28 eV) at the FCC site formed by three Pt atoms. This finally leads to a channel-like mobility of oxygen on the alloy surface. (C) 2004 Elsevier B.V. All rights reserved.