The electrodeposition of Pt100-xCux alloys is demonstrated using an electrolyte comprised of 1 mol/L H2SO4, 0.001 mol/L K2PtCl4, and 0.1 mol/L CuSO4. Film growth proceeds by facile Cu underpotential deposition occurring in parallel with Pt overpotential deposition, with the latter controlling the overall rate of alloy deposition. Pt and Pt100-xCux depositions are monitored in real time using an electrochemical quartz crystal microbalance. Pt deposition from 1 mol/L H2SO4 + 0.001 mol/L K2PtCl4 is diffusion limited at potentials below -0.150 V saturated sulfate electrode and occurs with close to 100% current efficiency. In the added presence of CuSO4, Pt100-xCux alloy deposition occurs, supported by the exothermic enthalpy of mixing. Monitoring the steady-state mass/charge ratio enables the composition of the growing film to be evaluated with submonolayer precision. For a given potential, the film composition so determined is in excellent agreement with the alloy composition determined for 1 mu m thick films using a combination of ex situ methods. The potential dependence of the alloy composition is described by an asymmetric regular solution model with an extremum for alloying enthalpy of - 18.9 +/- 0.2 kJ/mol. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3232216] All rights reserved.