The post-weld ageing behaviour of two commercial aluminium-lithium alloys, 2090 and 8090, was evaluated. Both a 2319 and 5556 filler material were used for preparing welds in Alloy 2090, while only the 5556 filler material was used with Alloy 8090. Welds with these filler metals were made at two dilution levels, nominally 35% and 65% dilution of the filler material by the base metal. The ageing behaviour of autogenous welds in each alloy was also evaluated. Post-weld heat treatments were performed at 325, 375 and 475 degrees F (163, 190 and 218 degrees C) for times up to 32 h. The ageing response was determined by performing hardness measurements at 4 h intervals. In general, dilution of the base metal by the filler metal reduced the ageing response of the combinations tested with autogenous welds in both 8090 exhibiting the highest peak hardness values. Ageing response curves for six base metal/weld metal/dilution combinations at two autogenous welds at three temperatures are presented.