We demonstrate that low energy ion scattering can be used to study the step-edge composition on vicinal single crystal surfaces. Employing the shadowing and focusing effects inherent to ion scattering, it is possible to perform site-specific composition measurements on single crystal surfaces over a wide temperature range. By combining these measurements with ion trajectory simulations we have extracted quantitative information concerning the step-edge composition. This technique has been applied to the Pt25Rh75(4 1 0) surface in the temperature range between 400°C and 700°C. The experiments reveal a much stronger Pt enrichment of the step edges than at the terrace sites. This difference between the step-edge composition and the terrace site composition is significantly larger than predicted by simple segregation models based on bond breaking.