This paper presents the performance of palladium-platinum core-shell catalysts (Pt/Pd/C) for oxygen reduction synthesized in gramscale batches in both liquid cells and polymer-electrolyte membrane fuel cells. Core-shell catalyst synthesis and characterization, ink fabrication, and cell assembly details are discussed. The Pt mass activity of the Pt/Pd core-shell catalyst was 0.95 A mg −1 at 0.9 V measured in liquid cells (0.1 M HClO 4 ), which was 4.8 times higher than a commercial Pt/C catalyst. The performances of Pt/Pd/C and Pt/C in large single cells (315 cm 2 ) were assessed under various operating conditions. The core-shell catalyst showed consistently higher performance than commercial Pt/C in fuel cell testing. A 20-60 mV improvement across the whole current density range was observed on air. Sensitivities to temperature, humidity, and gas composition were also investigated and the core-shell catalyst showed a consistent benefit over Pt under all conditions. However, the 4.8 times activity enhancement predicated by liquid cell measurements was not fully realized in fuel cells. Polymer electrolyte membrane (PEM) fuel cells face hurdles to commercialization in automotive applications in part because of the high cost of Pt-based catalysts used in the anode and cathode.