Electrode catalyst and electrochemical devices using the same

An electrode catalyst for fuel cells which comprises a conductive carbon, platinum supported on the conductive carbon in an amount of from 20% by mass to 70% by mass based on the mass of the catalyst, and oxygen bonded chemically to the conductive carbon and present in the range of from 0.7 to 3 in atomic ratio to the platinum. The present electrode catalyst can attain a high activity because the platinum crystallite diameter has been kept small even when the platinum is supported in a large quantity in the amount more than 20% by mass. The catalyst is useful in fuel cells, e.g., solid polymer electrolyte fuel cells.