A platinum alloy catalyst PtX, wherein the atomic percent of platinum in the bulk alloy is from 5 to 50 at %, the remaining being X, characterised in that the atomic percent of platinum at the surface of the alloy is from 10 to 80 at %, the remainder being X, provided that the at % of platinum at the surface of the alloy is at least 25% greater than the at % of platinum in the bulk alloy is disclosed.