The early stage mechanisms of grain refinement during ECAE of a single-phase aluminium alloy have been studied using the EBSD technique. It was found that, in addition to the formation of shear-plane cell bands and shear bands by "simple shear", the development of deformation bands due to orientation splitting contributed significantly to the refinement of microstructure. "Regular" slab-like deformation bands and "irregular" transitional bands were observed after the first pass; both developed boundaries of high misorientations. In the second pass, moderate orientation splitting took place within the deformation bands, although new deformation bands were not detected. With increased strains, fine scale orientation splitting tended to occur in local bands, generating high densities of new high misorientation boundaries. The crystallographic features of the different types of orientation splitting are examined.