We report a synthesis, activity and stability study of a dealloyed, highly active PtNi3 alloy nanoparticle catalyst for the oxygen reduction reaction (ORR) in acidic media. After activation by electrochemical dealloying of a PtNi3 precursor, the Pt-Ni nanoparticle catalyst (referred to as "dealloyed PtNi3") exhibits a 7-8 times higher Pt mass based activity and a 6-7 times higher Pt surface area specific based activity for ORR than pure Pt at comparable mean particle size. In addition, the long-term stability of the dealloyed PtNi3 was tested for typical fuel cell operating as well as more corrosive fuel cell start-up conditions. After 10000 voltage cycles between 0.5-1.0 V vs. RHE at 50 mV s(-1) the dealloyed PtNi3 catalyst still shows 4-5 fold increase in Pt surface area specific based activity compared with that for pure Pt. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.030201jes]