A pilot unit for the high-temperature irradiation of flowing reactants was designed for the beam port of a nuclear reactor. Radiation-thermal cracking of the n-heptane-hydrogen system was studied with molal H2/C7 ratios from 0 to 5 at 250 lb./sq. in., 600° to 750°F., 2 to 8 min. residence time, and up to 3,600 rep./min. Although conversions were low, the decomposition rate was significantly increased by radiation giving G values > 103. Product distributions were not significantly altered by radiation. These results, while different from low-temperature data, appear consistent with other published high-temperature results. Radiation yields were found to be reasonably linear with total dose from 0 to 14 krep., but a twofold increase was observed in passing from a molal H2/C7 ratio of 0 to 0.5.