The aluminum alloy 6061-T651 was a candidate material for several critical components of the proposed Advanced Neutron Source (ANS) research reactor. In order to determine the effects of irradiation on the mechanical properties of this alloy, irradiations have been conducted in the High Flux Isotope Reactor under conditions that are similar to those predicted for the ANS. Compact and tensile specimens of 6061-T651 aluminum were irradiated at a nominal temperature of 95°C to a thermal neutron fluence of approximately 8 × 1026 m-2 (<0.625 eV) for comparison with earlier data from material irradiated to a fluence of 1026 m-2. This lower fluence resulted in a small increase in the yield and ultimate tensile strengths, and a slight decrease in the elongation. However, the fracture toughness was not significantly affected at this fluence except at the highest test temperature of 150°C. Irradiation to the higher fluence resulted in a further increase in the yield and ultimate tensile strengths, and a further decrease in the total elongation. The fracture toughness showed little effect of the higher level of irradiation, except at the highest test temperature of 150°C, where the toughness showed an additional decrease. Weldments made with 4043 filler metal were irradiated to 2 × 1026 m-2. The yield and ultimate tensile strengths of the weld metal were much lower than the base metal in the unirradiated condition, but more than doubled after irradiation. The toughness of the unirradiated weld metal was very high, and it decreased significantly after irradiation, becoming similar to the unirradiated base metal.