Nano- and submicron-structured aluminium was produced by equal channel angular extrusion with a total strain of ~17. Large residual stress and strain energy were built up in the extruded metal and subsequent heat treatments were applied to investigate the stability of the nanostructures. X-ray diffractometry and transmission electron microscopy were performed to evaluate the microstructural changes in the nanograin metal. It was found that the nanostructures remained stable at temperatures up to 250oC. Above 250oC, changes in the major x-ray peak reflections became evident, suggesting substantial grain growth had occurred. Electron microscopy confirmed that at low annealing temperatures, the fine grain structures were stable and indeed became more equiaxed and well defined.