The hydrogenated terphenyl mixture HB-40 which is in use as a reactor organic coolant, has been irradiated at 350°C with 1.5-MeV electrons. Changes in composition and properties are reported. Decomposition proceeded at half the rate per unit dose observed previously during reactor irradiations where 62% of the absorbed energy was due to fast neutrons. This indicates a dependence of hydroterphenyl radiolysis on Linear Energy Transfer, whereby recoil protons produce 3.2±0.6 times as much decomposition as electrons. Some differences between the physical properties of electron-irradiated material and the properties of reactor-irradiated material were noted.