The surface tension of copper-nickel alloys was measured, both as functions of concentration and temperature, using electromagnetic levitation and the oscillating drop technique. The measurements covered the entire concentration range from c(Cu) = 0 to 100%, and a temperature range that included the molten as well as the undercooled regime. The results agree well with a model based on Butler’s formula for the surface tension and chemical potentials measured in the bulk liquid phase. This allows one to calculate the surface segregation of this system.