Platinum-cobalt (Pt-Co) alloy electrocatalysts were prepared on pretreated carbon cloth by electrodeposition from different plating baths with the aim of finding suitable plating bath conditions that could control the chemical composition of the Pt-Co alloy electrodeposits over a wide range (from 0 to 100 at％ Pt) and then to study the relationship of the deposit composition on the oxygen reduction reaction (ORR) electroactivity.The type of supporting electrolyte,the solution pH,Pt and Co concentrations of the plating bath and the current density used for electrodeposition all play a crucial role in the composition of the electrodeposited Pt-Co alloy.Pt contents in all the Pt-Co alloys deposited in H2SO4 are at a relatively high and narrow level (83 at％-97 at％ Pt),regardless of the electrodeposition conditions used.While the Pt contents in the Pt-Co alloys deposited in Na2SO4 could reach a very low and broad level (5 at％-97 at％ Pt),evidence of Co oxide formation is observed.Of the three supporting electrolytes studied,only NaCl effectively produces Pt-Co deposits over a wide range of compositions (8-90 at％ Pt)by controlling the Pt and Co concentrations of the plating bath and using the high enough current density.The results show a qualitative correlation between Pt-Co deposit composition and ORR activity,where Pt-Co alloy catalysts with ≥ 45 at％ Pt tend to yield higher ORR activities than the pure Pt catalyst,with the best ORR performance obtained from the Pt-Co alloy catalyst with 78 at％ Pt.