Commercial Al-Mg and Al-Mg-Si alloys were irradiated in the Los Alamos Meson Physics Facility (LAMPF) with 800-MeV protons. The irradiation experiment was carried out at a temperature between 40 to 100°C to a dose level of ≤0.2 dpa. Post-irradiation microstructure and mechanical properties of these alloys have been investigated. For comparison, a parallel investigation was conducted on unirradiated reference specimens as well as on unirradiated annealed (981 h at 100°C, corresponding to time at temperature during irradiation) specimens. It was found that the cold-work microstructure in the Al-Mg alloy survives the thermal annealing treatment but dissolves during irradiation already at a dose level of ≤0.1 dpa. In the Al-Mg-Si alloy magnesium silicide (Mg²Si) type precipitates also survive the thermal annealing but dissolve during irradiation at a low dose level of ≤0.1 dpa. Post-irradiation mechanical testing demonstrated that the strength of these alloys is drastically reduced because of irradiation. The observed irradiation softening can be understood in terms of the observed microstructural changes caused by irradiation.