Carbon supported Pt and Pt-Ni (1: 1) nanoparticles were prepared by reduction of metal precursors with NaBH4. XRD analysis indicated that only a small amount of Ni alloyed with Pt (Ni atomic fraction in the alloy about 0.05). The as-prepared catalysts were submitted to chronoamperometry (CA) measurements to evaluate their activity for the oxygen reduction reaction (ORR). CA measurements showed that the ORR activity of the as-prepared Ni-containing catalyst was higher than that of pure Pt. Then, their stability Was Studied by Submitting these catalysts to durability tests involving either 30h of constant potential (CP, 0.8 V vs. RHE) operation or repetitive potential cycling (RPC, 1000 cycles) between 0.5 and 1.0 V vs. RHE at 20 mV s(-1). After 30 h of CP operation at 0.8 V vs. RHE. loss of all non-alloyed Ni, partial dissolution of the Pt-Ni alloy and an increase of the crystallite size was observed for the Pt-Ni/C catalyst. The ORR activity of the Pt-Ni/C catalyst was almost stable, whereas the ORR activity of Pt/C slightly decreased with respect to the as-prepared catalyst. Loss of all non-alloyed and part of alloyed Ni was observed for the Pt-Ni/C catalyst following repetitive potential cycling. Conversely to the results of 30 h of CP operation at 0.8 V vs. RHE, after RPC the ORR activity of Pt-Ni/C was lower than that of both as-prepared Pt-Ni/C and cycled Pt/C. This result Was explained in terms Of Pt Surface enrichment and crystallite size increase for the Pt-Ni/C catalyst. (C) 2009 Elsevier B.V. All rights reserved.