Electrodeposition of ternary Pt100-x-yCoxNiy alloy films and the respective Pt binary alloys were examined and compared to the characteristics of the constituent elements. The potential dependence of alloy composition, structure and deposition current efficiency was determined using in situ electrochemical quartz crystal microbalance (EQCM) measurements and metallurgical analysis. The recently documented self-terminating electrodeposition of elemental Pt is disrupted by competitive adsorption and underpotential co-deposition of the iron group metals, whose onset overlaps with the H-upd region. Excess alloying enthalpy between Pt and the iron group metals supports the underpotential deposition of the iron group metals during on-going Pt deposition. The total iron group alloy content increases monotonically as the deposition potential becomes more negative. Below the reversible hydrogen potential the current efficiency drops with the onset of the proton reduction reaction. At more negative potentials the efficiency increases as the partial current for the Pt deposition and proton reduction reactions operate near diffusion control while the iron group metal deposition current increases monotonically. For ternary Pt100-x-yCoxNiy alloys grown from an electrolyte with [Ni2+]/[Co2+] = 1, Ni underpotential co-deposition is favored over Co, while in the overpotential regime anomalous co-deposition occurs with the Co content significantly exceeding that of Ni. (C) 2013 The Electrochemical Society.