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Electrocatalysts based on Pt-Ni alloys have received considerable interest in recent years owing to their remarkable activities toward the oxygen reduction reaction (ORR). Here, we report the synthesis of nanosized Pt-Ni octahedra with a range of controlled sizes and compositions in an effort to optimize their ORR activities. If we employed benzyl ether as a solvent for the synthesis, we could readily control the edge lengths of the Pt-Ni octahedra in the range of 6-12 nm and keep the Pt/Ni atomic ratio at around 2.4 by varying the amount of oleylamine added into the reaction system. If we adjusted the amount of Ni precursor, the atomic ratio of Pt to Ni in the Pt-Ni octahedra could be controlled in the range of 1.4-3.7 and their edge lengths were kept at 9 nm. For the catalysts with a Pt/Ni atomic ratio around 2.4, their specific ORR activities (per unit surface area) increased monotonically as the edge length increased from 6 to 12 nm. However, the mass activities (per unit mass of Pt) of these Pt-Ni octahedra showed a maximum value at an edge length of 9 nm. The specific and mass activities for the Pt-Ni octahedra with an edge length of 9 nm but different compositions both showed peak values at a Pt/Ni atomic ratio of 2.4., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)