We present a study of an ultrathin surface layer of platinum silicide formed on a Pt(lll) crystal as a result of surface segregation of Si and Ca trace impurities. The structure and composition of this gated with low energy He+ ion scattering, (LEIS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Si surface segregation onto Pt(lll) is thermally activated; annealing at temperatures between 750 and 1100 K results in the formation of a surface silicide. The highest Si coverage that can be reached is 0.4 monolayers; the Ca coverage at saturation is below 0.02 monolayers. The enthalpy of Si segregation is found to be -(105+/-30) kJ/mole. A well-ordered (root 19 X root 19)R23.41 degrees or /(3)(-2) (2)(5)/ structure is observed by LEEDS and STM; the amount of surface area covered with this structure is proportional to the Si coverage measured with LEIS. At low annealing temperatures up to 800 K, two domains coexist with/(3)(-2) (2)(5)/ and /(2)(-3) (3)(5)/ orientations, but only the first one is stable at Si saturation coverage. No large relaxations of substrate interatomic distances are detected upon formation of the overlayer.