The hydrogenated terphenyl mixture HB-40, which is of interest as a reactor coolant, has been irradiated in a loop in the NRX reactor under conditions simulating those that might pertain in an organic-cooled reactor. High Boiler decomposition products were removed by distillation and the recovered coolant was recycled to attain a stationary-state composition. Irradiation temperatures ranged from 250 to 375°C. Rates of coolant consumption and properties of the partly decomposed coolant mixture were determined. These are compared with the properties of the terphenyl coolant mixture, Santowax OM, irradiated under the same conditions. Important differences were noted between the properties of the stationary-state coolant mixtures and the properties of the starting materials. The HB-40 mixture appears suitable for use as a reactor coolant at temperatures up to at least 375°C. It has the advantage of remaining liquid under all conditions of use.