Low energy ion scattering has been used to perform site-specific composition measurements on Pt50Rh50(5 1 1) between room temperature and 1000 °C. The results show that the clean surface is strongly enriched in platinum. At all temperatures the Pt enrichment of the step edges is significantly larger than the Pt enrichment of the terraces, which can be attributed to the lower coordination of the step-edge atoms. A simple broken bond model is adequate to calculate the composition of the surface and the step edges at high temperatures. Between 650 and 800 °C a large decrease in Pt surface concentration occurs with increasing temperature. Two possible explanations are discussed here. The first is a bulk phase demixing at temperatures below 760 °C, which would lead to enhanced segregation due to the local increase of the Pt concentration in the sub-surface region. The second possible explanation is a surface phase transition from a missing row reconstruction below 650 °C to an unreconstructed surface above 800 °C. Here the enhanced segregation at lower temperatures is due to the decreased orientation of the surface atoms in the reconstructed phase. The influence of oxygen on the segregation is studied by measuring the surface and step-edge compositions after the adsorption and subsequent removal of oxygen. Following this preparation, the step-edge and surface Pt concentrations are equal to each other within the experimental accuracy. The results show weak Rh segregation on the oxygen treated surface, in contrast to the strong Pt segregation found on the clean surface.