Abstract Molybdenum was neutron-irradiated near 80 °C to doses of 7.2 × 10 −5, 7.2 × 10 −4, 7.2 × 10 −3, 0.072 and 0.28 dpa. Post-irradiation examination included electrical resistivity and tensile properties measured at room temperature. Microstructure of irradiated specimens was examined by TEM and the defect cluster density and cluster mean size were characterized. Measurements of electrical resistivity and cluster density showed sublinear defect accumulation behavior. The mean size of visible defect clusters increased with increasing dose. Yield stress decreased at 7.2 × 10 −5 and 7.2 × 10 −4 dpa, then increased significantly with increasing dose up to 0.072 dpa and saturated. It appeared that there was a transition in hardening from weak obstacles to strong obstacles. It is suggested that the formation of sessile defect clusters in neutron-irradiated Mo is mainly associated with diffusive nucleation and growth rather than in-cascade clustering.