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The electrodeposition of copper–tin alloy thin films was studied as a potential application for on-chip interconnection, where Cu–Sn alloys may be less susceptible to failure caused by electro- and stress-migration than pure copper. Alloys with variable Sn content were plated from acidic copper sulfate electrolytes by polarizing copper deposition into the region where Sn deposition became possible. The effective polarization was demonstrated by means of several halogen–polyether pairs, with Br–EPE (poly(ethylene glycol)- block -poly(propylene glycol)- block -poly(ethylene glycol)) pair exhibiting the strongest polarization. Alloy content of Sn between 0 and 7 at% was obtained above the reduction potential of Sn 2+ , which can be attributed to UPD mechanism. Higher Sn content of up to 20 at% was achieved when Cu deposition was suppressed below the reduction potential of Sn 2+ by the combination of Br − and EPE. A positive correlation between Sn content and concentration of Sn 2+ in the electrolyte was observed. At low rotation speeds of a disk electrode ( i.e. 25 rpm versus 100 rpm) the tin content in the alloy was higher, possibly due to stronger suppression of copper deposition at low rotation speeds. The relationship between Sn content and the applied current density was specific to the employed additive chemistry.