Mg-Gd represents a promising light hardenable alloy with a high creep resistance even at elevated temperatures. A disadvantage of this alloy consists in a low ductility. Recently it has been demonstrated that ultra fine grained (UFG) metals with grain size around 100 nm can be produced by high pressure torsion (HPT). A number of UFG metals exhibit favourable mechanical properties consisting in a combination of a very high strength and a significant ductility. The HPT processing creates a high density of defects, especially dislocations, in UFG material. These defects play key role in the specific physical properties of UFG materials. Defects studies of UFG materials are, therefore, very important for understanding of their unusual properties. Positron lifetime (PL) spectroscopy is one of the most powerful techniques for investigations of open-volume defects. In the present work we employed PL spectroscopy for defect studies of UFG Mg-Gd alloys prepared by HPT. The PL investigations were combined with transmission electron microscopy, X-ray diffraction, and microhardness measurements. At first, the microstructure of the as-deformed specimens was characterized. Subsequently, the specimens were isochronally annealed and the development of microstructure with increasing temperature was investigated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)