Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy Pt x Y is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model Pt x Y nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mg Pt −1 at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of Pt x Y over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core. P olymer electrolyte membrane fuel cells (PEMFCs) hold the potential to provide a zero-emission power source for future automotive applications. However, their widespread commer-cialization is hindered by the high loadings of Pt required to catalyse the oxygen reduction reaction (ORR) at the cathode 1–4 . An order of magnitude increase in ORR mass activity (that is, current density per unit mass Pt) over state-of-the art commercial pure Pt catalysts would bring the precious-metal loading in fuel cells to a similar level to that used for emission control in internal combustion engines 3,5 . Some alloys of PtX (X = Co, Ni, Cu) show higher ORR activity in comparison to pure Pt (refs 1–4), but typically their long-term performance is compromised by their poor stability against dealloying 6,7 . In recent years, progress has been made towards the stabilization of Pt-based catalysts 8–12