The effect of helium production during neutron irradiation on the high temperature tensile properties of copper and dispersion strengthened (DS) copper alloys was investigated in neutron-irradiated specimens containing different levels of boron. Sheet tensile specimens were irradiated at temperatures of 90°C, 150°C and 300°C to doses of 0.2–1 dpa. Considerable embrittlement (<3% total elongation at 400°C) was detectable in pure copper at He concentrations above 80 appm. The DS copper alloys tested under the same conditions showed higher resistance to helium embrittlement. Samples of DS copper alloy saturated to 60 appm helium on cyclotron also demonstrated high resistance to helium embrittlement. The present results indicate that under fusion energy conditions, where a high helium accumulation rate of ∼10 appm/dpa is expected, helium embrittlement may limit the maximum temperature of copper alloys for structural applications to ⩽ 400°C.