Al-2.3 wt % Li-1.8 wt % Mg-0.15 wt % Zr alloys with 0.1 and 0.45 wt % Cu were studied. The materials were cast, homogenized at 530 degrees C for 24 h, extruded at 400 degrees C with a reduction ratio of 30 : 1, solution treated at 528 degrees C for 0.5 h and then artificially aged at 190 degrees C to an overaged condition. The concerted application of light-, transmission electron- and scanning electron microscopy together with short rod fracture toughness measurements allowed the influence of low level Cu together with 0.15 wt % Zr on the fracture tough ness of this alloy system to be studied. It was found that the addition of 0.1 wt % Cu together with 0.15 wt % Zr not only affected the delta' precipitate nucleation and their size and size distribution through Li-Cu-v triple complexes but also affected the beta' dispersoid nucleation through Zr-Cu-v triple complexes. This in turn affected the size and size distribution of the delta'/beta' duplex precipitates. This causes an increase in the fracture toughness in underaged material artificially aged at 190 degrees C. This effect was probed in an SEM fractographic investigation and by mechanical testing.