Pd-Co alloys electro catalysts were synthesized for the oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells by a new ultrasonic spray method. This allows for the first time the preparation of reproducible small particle size with dominant dimensions between 2.5 and 6.5 nm for all samples of the Pd-Co electro catalysts. This method is also based on less time consuming and steps than the classical methods. The physico-chemical properties of Pd-Co electro catalysts from different compositions were characterized. Samples of Pd/C, Pd5Co1/C, Pd3Co1/C, Pd2Co1/C, and 5 Pd1Co1/C where synthesized and Pt/C for comparison. The samples of Pd-Co alloys prepared here are substitution solid solutions where some atoms of Pd are replaced by Co atoms. Based on experimental bending energies, the main composition of the samples is found to be metallic Pd and Co. The physic-chemical composition and properties of the alloys were correlated to the ORR electro catalytic activity in an acid medium. A volcano plot was observed between the variation of the mass activity and the Pd content or Pd-Pd distance of the Pd-Co alloys. The optimum value of the mass activity was obtained for Pd3Co. (C) 2013 Elsevier Ltd. All rights reserved.