The minimum grain size obtainable in an Al-0.1%Mg submicron grained (SMG) alloy, subjected to cryogenic plane strain deformation, and its subsequent stability during room temperature deformation have been investigated. A decreasing steady state grain size was obtained with reducing deformation temperature. However, a true nanocrystalline grain structure was not obtained even at 77 K with the high angle boundary spacing only approaching the nanoscale in the sample normal direction. The cryogenically deformed material was unstable on subsequent deformation at room temperature and underwent rapid dynamic grain growth. Dynamic grain coarsening is shown to limit the minimum grain size achievable in an SPD process, even under cryogenic conditions.