Using equal channel angular extrusion (ECAE) a high purity Al-0.13wt.%Mg alloy has been deformed at room temperature up to effective strains of 10. The evolution of the deformed state and the mechanisms for the formation of submicron grains, during severe deformation, has been studied by high-resolution electron backscattered diffraction (EBSD). At low strains, ECAE produces microstructures similar to those found in rolling. At medium strains, continuous high angle boundaries are formed by the collapse of transition bands, which rotate towards the shear direction forming a lamella structure. Narrow bands of fine grains also form in unstable crystal orientations. Increasing the strain reduces the separation of the continuos lamella boundaries until they approach the subgrain width and at very high strains (epsilon = 10). These thin ribbon grains then break up in to shorter segments resulting in a microstructure consisting of submicron grains with an aspect ratio of 3. A significant proportion of larger fibrous grains are still retained even at this strain.