We present results of the surface composition of the topmost layer of three low index planes of Pt xNi 1-x single crystal alloys: Pt 10Ni 90(100), Pt 25Ni 75(111) and Pt 50N 50(110). The alloys were investigated by ion scattering spectroscopy (ISS). We observed Pt enrichment of the surface layers caused by preferential sputtering. After exposing the surfaces to oxygen, adsorbate induced segregation of Ni was measured. Annealing the sputtered alloys results in a site specific change in surface composition. We found Pt enrichment for the (111) surface and Ni enrichment for the (110) surface. The surface of Pt 10Ni 90(100) showed a Pt concentration close to the bulk composition. Work functions were determined by ultraviolet photoelectron spectroscopy (UPS). The UPS results support the ISS results of the surface composition. The results of thermodynamic calculations are discussed with respect to the experimental results on surface segregation.