Electrochemical co-deposition of CoxPt1-x alloys is demonstrated at potentials positive to that required to deposit elemental cobalt (underpotential, or "induced,'' co-deposition). Co-deposition is attributed to the negative enthalpy of CoxPt1-x alloy formation. A regular solid-solution model is used to describe the dependence of the film composition on the deposition potential. X-ray diffraction of films grown on Cu(100)parallel to Si(100), as well as poly- Au parallel to SiO(2)parallel to Si, reveal the formation of a randomly oriented CoxPt1-x face-centered cubic alloy for x > 0.3. A transition to the hexagonal close-packed phase is evident at higher cobalt concentrations (x > 0.9). (C) 2004 The Electrochemical Society.