Commercially pure aluminum (1100 grade) in the annealed condition was irradiated to fast fluences in the range 1 to 3 × 1026 neutron/m2 (E > 0.1 MeV) and to similar thermal fluences at temperatures of 318 to 328 K, and was then tensile tested at temperatures between 298 and 643 K. This irradiation doubled the ultimate tensile strength and more than tripled the flow stress at all test temperatures. The work hardening exponent was severely reduced and there was a large loss in ductility. Strengthening is shown to be due to a fine precipitate of transmutation-produced silicon and an associated dislocation structure. At test tempera-tures below about 423 K the fracture mode was transgranular. Above 473 K grain boundary cavities were observed, the fracture mode become predominantly intergranular, and ductil-ity was further reduced. Unirradiated specimens containing cyclotron-injected helium showed no change in strength but displayed a loss in ductility at elevated temperatures. Concurrently holes were formed on the grain boundaries. Embrittlement in the neutron-irradiated specimens arises from two sources. One is through the defect structure which reduces the work-hardening exponent. The other is an additional effect at elevated temper-atures involving grain boundary failure by cavity growth and coalescence. Helium encour-ages cavity nucleation, the degree of cavitatlon increasing with increasing tensile strength.