Recent experiments have indicated that some organic materials such as diphenyl may be sufficiently resistant to radiation damage to perform satisfactorily as coolants for nuclear reactors. These materials offer such advantages as low vapor pressure at high temperatures, lack of induced radioactivity, and low corrosion attack on fuel and structural materials. A collection of data on vapor pressure, density, specific heat, viscosity, thermal conductivity, and heat transfer film coefficient for diphenyl is presented as an aid to the reactor designer. Some experimental data on melting points and viscosities of mixtures of diphenyl with higher polyphenyls are also given.