Abstract Scanning tunnelling microscopy on PtRh(100) (molar bulk composition 1 : 1) has revealed the possibility of direct determination of the surface of this system. During measurements at low tunnelling resistance ( <500 kЩ ), the Pt and Rh atoms appear with a clearly observable apparent height difference of more than 20 pm. No long range ordering has been found. Variation of the sample preparation method and comparison of STM and Auger electron spectroscopy measurements led to the conclusions that there is a preferential surface segregation of platinum, that rhodium atoms are the ones with the highest apparent height, and that there is limited tendency of clustering on the surface. Furthermore, it was found that platinum atoms preferentially populate the step edges on this crystal surface.