Ni-Mo based "Hastelloy" alloys are structural materials for the accelerator-driven molten-salt reactor, in which they are subjected to intense radiation damage. This causes a complete alteration of the microstructure. Since atomic collisions are responsible for inducing, modifying and activating the kinetic processes which alter the microstruture, high energy electron or proton irradiation is suitable to simulate the microstructural changes. A "Hastelloy" alloy with the nominal composition Ni - 11.7 Mo -6.7 Cr - 1.5 Fe - 0.8 Al - 0.5 Ti - 0.5 Mg - 0.5 Nb - 0.15 Si - 0.05 Y (wt. %) was investigated in this study. The material was homogenized at 1100°C for 1 h and aged at 675°C for 50 h. Scanning electron microscopy (SEM) studies demonstrate decoration of the grain boundaries by large Ni-Ti-Si precipitates with a high content of Y (up to 30 at.%). Local-electrode atom-probe (LEAP) tomographic analyses reveal additional small precipitates with a cylindrical morphology. The composition of these precipitates is 73.8 Ni - 14.8 Al - 3.7 Ti - 3.6 Mo - 1.7 Cr (at.%) and small amounts of Mn, Fe, Si and Nb. This composition corresponds to an intermetallic phase with an A3B structure. Additionally transmission electron microscopy (TEM) analyses show that the precipitates have a mean diameter of 22 nm and are coherent with the fcc matrix. The number density of these precipitates is about 2x1021 m-3. Electron-diffraction patterns from these precipitates exhibit superlattice reflections corresponding to the ordered L12 structure. For studying the microstructural changes during irradiation, the alloy was irradiated with 10 MeV electrons at 650°C for 700 h. The dose was 2x10 -3 dpa (displacement per atom). The large precipitates at the grain boundaries remain unchanged by the electron irradiation. The small precipitates with the cylindrical morphology, however, are no longer visible. Instead, new small precipitates with an irregular morphology are observed. The number density of these new precipitates is of the same order as that of the A3B precipitates before irradiation. The L12 diffraction spots disappear completely, instead diffuse diffraction spots are observed at 〈1 1/2 O〉fcc, which are attributed to short range order (SRO). An ordering transformation from L12 to 〈1 1/2 O〉fcc requires a change of the atom positions. SRO with 〈1 1/2 O〉fcc superstructure reflections have been found in Ni 3Mo or in Ni4Mo precipitates in Ni-Mo alloys after thermal treatment. However, the content of Mo in the investigated "Hastelloy" alloy is significantly lower than that of these alloys. In both cases the ordering transformation requires changes of atom positions, which are induced by thermally activated transport or by radiation-induced diffusion, respectively. In the present case the irradiation damage may lead to the formation of metastable phases.