In a major irradiation program aimed at qualifying a tube material for the absorber rods of a pebble-bed high temperature gas cooled reactor (HTGR). tensile and creep test specimens of the nickel-chromium based, solid solution strengthened, high temperature alloy Hastelloy X were irradiated at 673 or 873 K in six reactor experiments to accumulate thermal neutron fluences between 1.2 × 10²² and 3.1 × 10²⁵ m^-2. In addition. 110 μm thick foils were homogeneously implanted with helium from a cyclotron to helium concentrations of 10, 150, and 480 atomic parts per million (appm), corresponding to helium contents expected in the high fluence neutron irradiation experiments. Tensile and creep rupture tests were performed with irradiated and implanted samples as well as virgin controls in the temperature range 300 to 1123 K. The effect of irradiation is a hardening and a reduction in ductility at all temperatures due to irradiation enhanced precipitation compared to the solution annealed controls. In addition, a strong embrittlement was observed at high temperatures (T > 800 K) increasing with increasing neutron fluence and implanted helium concentration. At the highest test temperature (1123 K), catastrophic grain boundary embrittlement appeared at all neutron fluences down to 1.2 × 10²² m^-2 corresponding to only 10^-3 dpa and 0.04 appm helium. In order to get some microstructural indications about the possible reasons for the embrittlement—helium or irradiation-induced enhanced solute segregation to grain boundaries or both, the microstructure was characterized by transmission electron microscopy (TEM), but no conclusive decision over the embrittling mechanism at such low fluences could be made.