In the present investigation deformation microstructures forming during friction stir welding (FSW) of the aluminium. aerospace alloy 2024 T351 have been examined. Electropolished specimen were examined by means of fully automatic electron back-scattered diffraction (EBSD) analysis in a JEOL 840 scanning electron microscope (SEM) equipped with NORDIF EBSD-hardware and Channel 4.2 EBSD-software. The EBSD-technique has proved to be a useful tool for characterisation of the deformation microstructures forming during friction stir welding. Significant local variations in the deformation microstructure are observed in the weld nugget region of the 2024 T351 alloy in the as-welded condition. The grain structure shows characteristics typical of different stages of dynamic recrystallisation. In some regions a fully recrystallised microstructure is observed, consisting of very fine 1-2 mum equiaxed grains mainly separated by high angle grain boundaries. Other regions contain grains that are partially recrystallised with a relatively large fraction of subgrains being observed. After high temperature annealing at 500degreesC for 60 minutes it is evident that abnormal grain growth has occurred in the weld nugget. This microstructure contains a mixture of grains ranging from 4 mum. up to about 170 mum.