Combined copper and magnesium additions have been made to a base Al-1.7wt%Li alloy to investigate their effect on the microstructural stability of Al-Li-Cu-Mg alloys. Alloys have been given a standard age of 24h at 150 degrees C, followed by a second ageing treatment of 1000h at 70 degrees C to stimulate low temperature exposure under service conditions. Exposure at 70 degrees C resulted in a decrease in fracture energy and an increase in proof stress. The magnitude of these changes in mechanical properties were larger than those observed in binary Al-Li and ternary Al-Li-Cu alloys. TEM, DSC and electrical resistivity measurements have been used to establish the microstructural changes and precipitation kinetics that cause this increased level of exposure embrittlement. It is proposed that in Al-Li-Cu-Mg alloys Cu/Mg co-clusters or GPB zones form during the 150 degrees C pre-age and these act as nucleation centres for delta' (Al3Li) formation during the 70 degrees C exposure. This produces enhanced kinetics of delta' precipitation and therefore a larger volume fraction of delta' forms during the exposure treatment.