The effect of neutron irradiation (1.7 × 10 17−8.15 × 10 19 n/cm 2, fast and 4.6 × 10 17−2.2 × 10 20 n/cm 2, thermal) on the mechanical properties and grain growth of pure copper, Cu + 2 at % Al and Cu + 2 at % Mg in the temperature range 25–900 °C was studied. Irradiation produced changes in the mechanical properties with subsequent recovery at elevated temperatures. At temperatures higher than those of recovery appreciable differences in elongation to fracture of the irradiated and unirradiated alloys were observed. A linear relationship between the drop in ductilities and the thermal neutron integrated doses was found at 600 °C. The differences in ductilities and in grain growth of the irradiated and unirradiated materials were explained by the presence of helium and/or lithium atoms generated through the 10B (n, α) 7Li reaction.