Electrochemical codeposition of a series of face-centered cubic NixPt1-x alloys is demonstrated (0.1 < x < 0.95). The alloy composition is a monotonic function of potential. The Pt-rich NixPt1-x alloys are formed at potentials positive to that required to deposit elemental Ni. Codeposition is ascribed to the negative enthalpy of NixPt1-x alloy formation that proceeds via a Ni underpotential deposition reaction in concert with Pt deposition. Interestingly, this process occurs at higher Ni underpotentials than anticipated based on extrapolated literature data from thermochemical measurements and ab initio calculations of alloy formation. In contrast, Ni-rich NixPt1-x alloys are produced at Ni overpotentials although the films are formed under conditions where pure Ni deposition is otherwise kinetically hindered. The alloy composition corresponding to the transition from underpotential to overpotential deposition is a function of the PtCl4/NiCl2 electrolyte composition. The films were found to be bright and specular over the full range of compositions studied (grain size < 10 nm ). Atomic force microscopy yielded root-mean-square roughness values on the order of 5 nm for Ni-rich deposits up to 2.5 mu m thick. (c) 2007 The Electrochemical Society.