The proposed experiment will study the deformation of a two phase Zr-2.5Nb alloy at cryogenic temperatures. It forms part of our ongoing long-term project to understand the deformation mechanisms of zirconium based alloys. It will explore the potential for twinning to occur in the alpha phase, when slip is suppressed by cooling. The impact of different amounts of interstitial oxygen will be examined, allowing separation of the effect of O on slip and twinning.