Sn was deposited onto a Cu substrate by the contact immersion processes, where a Cu substrate was dipped into a Sn(2+)-citrate solution, and was directly or electrically contacted to metallic Sn dipped in the same solution. Sn was deposited at underpotential and the deposition kinetics followed a parabolic rate law. The deposited Sn did not exist as elemental Sn films but formed Cu-Sn intermetallic phases. The growth of the Cu-Sn intermetallic layer is continued by the underpotential deposition of Sn onto this layer, and its rate is determined by the solid-state diffusion of Sn and Cu atoms through the intermetallic phase films.