The reactions of superthin Pb films on a Cu (111) surface with respect to the coverage of Pb and annealing have been studied by synchrotron radiation photoemission. The submonolayer Pb atoms deposited at room temperature are distributed on the Cu (111) surface as two-dimensional (2D) islands. Annealing to 200℃ gives rise to Pb-Cu surface alloy formation. Analyses show that the surface alloy occurs only in the first layer of the Cu (111) surface. As a surfactant, Pb can promote 2D layer-by-layer growth of thin films on Gu (111), but the Pb-Cu surface alloying may have an unfavorable effect on the activation process.