Activity–composition relations in Pt–Cr and Pt–V alloys at 1500°C were determined from equilibration of these alloys with Cr2O3and V2O3, respectively, in Co2–H2atmospheres of known oxygen partial pressures, followed by quenching to room temperature and microprobe analyses of the quenched alloys. The activities of Cr and V were calculated from known standard free energies of formation of Cr2O3and V2O3from the respective elements, and the oxygen pressures were calculated from the C–H–O equilibrium in the gas phase. Both alloy systems show strong negative deviations from Raoult