We report on synthesis-structure-activity-stability relationships Of Pt3CO nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). We have synthesized Pt3Co alloy electrocatalysts using liquid impregnation techniques followed by reductive annealing at high and low temperatures. We have performed detailed structural X-ray diffraction (XRD)-based structural characterization (symmetry, lattice parameters and composition) of individual Pt-Co alloy phases before and, importantly, after electrochemical rotating disk electrode (RDE) measurements. This enables us to directly evaluate the corrosion stability of various Pt-Co alloy phases under typical fuel cell cathode conditions. Pt3CO prepared at low annealing temperatures (600 degrees C) resulted in multiple phases including (i) a disordered face-centered cubic (fcc) Pt95CO5 phase and (ii) an ordered face-centered tetragonal (L1(0)) Pt50Co50 phase; high temperature annealing (950 C) resulted in a single ordered primitive cubic (Ll(2)) Pt3Co phase. The ordered alloy phases in both catalysts were not stable under electrochemical treatment: The ordered face-centered tetragonal (fct) phase showed corrosion and dissolution, while the ordered primitive cubic (Ll(2)) Pt3Co phase transformed into a disordered structure. The ordered primitive cubic structure exhibited higher resistance to sintering. Low annealing temperatures resulted in higher Pt surface-area specific activities for ORR. Kinetic Tafel analysis confirmed a general shift in the formation potential of oxygenated surface species, such as Pt-OH, for both alloy catalysts. Reduced OH coverage alone proved insufficient to account for the observed activity trends of the two alloy catalysts. (c) 2006 Elsevier Ltd. All rights reserved.