Co-Rh bimetallic model catalysts were prepared as thin films on a Mo(1 0 0) substrate and as dispersed clusters on a SiO2 support under ultrahigh vacuum conditions. The surface composition and electronic structure of the Co-Rh thin films on the Mo(1 0 0) substrate were characterized with low energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS). Co-Rh thin films form a stable alloy between 700 and 1000 K with substantial surface enrichment of Co. Annealing a 1:1 Co-Rh thin film at 700 K for 30 min yields an alloy with 79% (atomic concentration) of Co in the outmost surface layer. The surface composition and morphology of the Co-Rh dispersed clusters on the SiO2 support were determined by LEIS, XPS and transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDX). The Co and Rh atoms mixed to form bimetallic clusters between 700 and 800 K with Co segregated to the cluster surface. EDX spectra provided direct proof for the formation of a Co-Rh alloy. TEM images showed that the Co-Rh clusters were highly dispersed on the SiO2 support and the average particle size of the bimetallic clusters increased monotonically with the total Co fraction. (C) 2010 Elsevier B.V. All rights reserved.