The mechanisms of grain refinement during severe plastic deformation have been studied, by comparing the microstructure evolution in an AA2219 aluminium alloy, containing Al3Zr nanoscale particles, with that in a dilute Al-3%Cu alloy deformed identically by equal-channel angular extrusion (ECAE) at 250degreesC to a maximum strain of similar to12. Transmission electron microscopy (TEM) was used on the AA2219 alloy to reveal the misorientations of deformation-induced boundaries. Microstructural evolution in the Al-3%Cu alloy was studied by electron-back scattering diffraction (EBSD) orientation mapping. It was shown that the mechanism of grain refinement in the AA2219 alloy is continuous dynamic recrystallization (CDRX) consisting of two main elementary processes. In the initial stages of plastic deformation, the formation of three-dimensional arrays of low-angle boundaries (LABs) takes place. Further strain results in increasing misorientation of these boundaries providing their gradual transformation into high-angle boundaries (HABs). A fully recrystallized structure with an average grain size of similar to0.9 mum is evolved after a total strain of similar to12. In the dilute Al-Cu alloy the evolution of ultrafine grains with an average size of similar to6 mum is attributed to the formation of deformation bands outlined by HABs and extended medium to high-angle boundaries at moderate strains. The subdivision of these deformation bands into fine grains rarely occurs through the mechanism of geometric recrystallization (GRX). In this alloy the main contribution in the grain refinement gives CDRX occurring within fibrous structural features. At epsilonsimilar to12, a partially recrystallized structure is formed in the Al-3%Cu alloy.