The surface states of supported PtNi nanoparticles treated at alternating O-2 and H-2 atmosphere were studied by X-ray photoelectron spectroscopy. Reversible structural changes at the surfaces of the supported PtNi nanoparticles in response to reaction gases were observed, showing NiO-rich surface in oxidizing gases and Pt-rich surface in reducing gases. The dynamic behaviors can be attributed to two opposite gas-driven mass transport processes at bimetallic particle surfaces, including surface segregation of Pt at the bimetallic particles in H-2 and encapsulation of the particles by NiO in O-2. The similar surface structural changes can be observed in other bimetallic catalyst systems, as exemplified in supported PtCu and PtCo nanoparticles. (C) 2009 Elsevier B. V. All rights reserved.