An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of "nano-engineered" catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt-Rh alloy between 873 and 1073 K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh(2)O(3) oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt-Rh as 236 +/- 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core-shell structure Pt-Rh nanoparticles. (C) 2011 Elsevier B.V. All rights reserved.