The present work is an attempt to contribute the current understanding of the operating mechanisms responsible for the development of ultrafine grains during severe plastic deformation (SPD) in aluminium. Equal channel angular pressing (ECAP) at room temperature has been applied to a commercial Al-Mg-Si alloy. Route A was used in all pressings and the first two passes were studied in most detail. Advanced characterization methods such as FEG-SEM with a state-of-the-art microdiffraction unit has been applied when characterizing samples carefully prepared from different positions along certain flow paths in the process shear zone. Intrinsic strain measurements are done in parallel in order to describe the actual strain tensor in each position studied. Detailed information on phenomena involved in the grain break-up mechanisms has been obtained by high resolution EBSD data collected through the deformation zone. The microstructural development seems to be dominated by deformation banding and may be explained in terms of the LEDS theory.