Six Pt-Ru/carbon nanocomposites have been prepared, using five different Pt,Ru-bimetallic precursors as sources of metal. Nanocomposites prepared from precursors lacking phosphorus contain Pt-Ru nanocrystals that are highly dispersed on the carbon support, as expected. However, nanocomposites prepared from precursors possessing phosphorus contain a mixture of face-centered-cubic Pt-Ru alloy nanocrystals and primitive-cubic nanocrystals of an interstitial ternary metal phosphide phase identified as PtRuP2. Nanocomposites containing substantial amounts of nano-PtRuP2 perform as well as a commercial Pt-Ru/carbon nanocomposite in the role of an anode catalyst in direct methanol fuel cells. Although the exclusion of metal phosphide phases in Pt-Ru nanocomposite syntheses is best achieved by eliminating all sources of phosphorus, the presence of PtRuP2 in such natiocomposites does not poison methanol electrooxidation. Investigation of the synthesis and electrocatalytic reactivity of pure PtRuEx (where E denotes a main-group heteroelement) phases is suggested.