The phase formation in the ternary Ni/Yb/Si system was studied for Ni-Yb alloy and interlayer structures on Si(100) substrates using in situ x-ray diffraction measurements. Yb was treated as an alloying element in the Ni-Si system with Yb concentrations varying between 0 and 40 at. % of the Ni concentration. Independent of the initial structure of the sample, a Ni-Si or Ni-Yb compound was detected first which suggests that Ni is the dominant diffusing species during the solid state reactions. No pure Yb silicides were identified but a ternary phase (YbNi2 Si2) formed in all samples after the Si atoms became mobile. Information about the distribution of the phases throughout the thin silicide film was obtained using ex situ Rutherford backscattering analysis. Independent of the Yb concentration, the NiSi phase formed at the substrate interface. As a result, the immobile Yb atoms shifted toward the sample surface and no detectable amount of Yb atoms was left at the Ni-silicide/Si interface after annealing. © 2010 American Vacuum Society.