This investigation established the effects of bulk immiscibility on the bulk and surface structure of FePd catalyst particles supported on graphite. By use of Mossbauer effect spectroscopy, X-ray diffraction and gas-adsorption microcalorimetry, it was shown that iron-rich FePd/Grafoil catalyst particles consist of thermodynamically stable segregated states of metallic iron and FePd alloy in accordance with the bulk immiscibility of the two phases. This indicates that manipulation of metal stoichiometry in bimetallic catalyst particles with weak metal support interaction provides another means to modify particle structure and hence catalytic chemistry.