The evolution of microstructure and microhardness was studied in a commercial 5483Al-5Mg alloy processed by high pressure torsion (HPT) under a pressure of 6.0 GPa up to 10 turns. Significant grain size refinement was observed even after 1/4 turn and additional processing led to a further grain size reduction and a shift in the distribution of grain boundary misorientation angles towards higher values.An essentially fully homogeneous microstructure was reached after 10 turns with a final grain size of ~70 nm, a saturation Vickers microhardness of Hv 240 which was attained at and above equivalent strains of ~150, a relatively narrow grain size distribution and a fraction of ~80% of high-angle grain boundaries. Analysis shows the Hall-Petch plot deviates from the conventional linear relationship for samples processed through small numbers of turns but after 3 or more turns there is a direct correlation between the results obtained in HPT processing and coarsegrained samples.