Electrochemical alloying of Cu substrates through a reduction-diffusion method was investigated using an ionic liquid, trimethyl-n-hexylammonium bis[(trifluoromethyl)sulfonyl]amide, as a solvent for an electrolytic bath. The use of the ionic liquid made it possible to raise the processing temperature beyond 100 degrees C and form the Cu-Sn layers faster than with an aqueous media. The layers obtained from a Cu thin layer under a potentiostatic condition were silver-gray speculum metal composed of Cu6Sn5, Cu3Sn, and Cu10Sn3 intermetallic phases, while those prepared under a galvanic contact condition involved a beta-Sn phase containing a trace amount of copper. The formation of each Cu-Sn phase is discussed in terms of alloy formation thermodynamics. (c) 2007 The Electrochemical Society.