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The kinetics of the interdiffusion of Co adatoms on a Pt(111) surface was studied by Auger electron spectroscopy at various temperatures with Co coverage near 1 ML. After annealing, the Co concentration decreases dramatically before it approaches a constant value. The Co–Pt alloy is confined in the few topmost layers. Activation energies and pre-exponential diffusion coefficients are obtained from an Arrhenius plot. The values are 0.90 eV and 6.6×10−11 cm2/s, respectively. They are independent on the coverage in this ultrathin system. After annealing the 1 ML Co ultrathin film on a Pt(111) surface at 600 K, Co atoms diffuse into the first two monolayers only. Both the topmost and the second layers contain 50% of Co atoms. In addition, ultraviolet photoelectron spectroscopy was used to monitor the alloy formation of Co/Pt(111) in the annealing process.