The carbon supported palladium-copper alloyed catalysts (Pd-Cu/C) were synthesized by a facile two-step method with different molar ratios of Cu to Pd (Cu/Pd), namely, Cu/Pd = 2.0, 3.0, 4.0, and 5.0. The Cu nanoparticles were obtained by microwave assisted method, and followed by galvanic substitution of Pd on Cu nanoparticles. The as-prepared Pd-Cu/C exhibited mainly fcc phase with the average crystallite sizes ranging 4.0 similar to 4.5 nm by varying Cu/Pd ratios of 2.0 similar to 5.0. The degree of Pd-Cu alloying was significantly influenced by Cu/Pd ratios, which ultimately affected the activity toward oxygen reduction reaction (ORR) and stability of Pd-Cu/C. The best ORR activity with good stability was obtained for the Pd-Cu/C prepared with Cu/Pd = 4.0 owing to the formation of fcc PdCu alloy structure. The half-wave potentials (E-1/2) reached 0.76 and 0.86 V (vs. RHE) in HClO4 and KOH solutions, respectively, which were comparable to those of commercial Pt/C, while the E-1/2 values remained virtually unchanged after 1000 cycles. However, the limiting current density decreased by 12.7% in HClO4, resulted in 58.5% loss of electrochemical surface area due mainly to Pd-Cu dealloying. The maximum power densities of 315 and 181 mW.cm(-2) were achieved with Pd-Cu/C at 0.10 mg Pd.cm(-2) in the H-2/O-2 and H-2/air single cells, respectively. (C) 2019 The Electrochemical Society.