Magnesium alloys are particularly attractive due to their low density but they generally exhibit moderate mechanical properties at room temperature and due to the hcp structure, conventional fanning techniques may be difficult to apply. A way to partly overcome these difficulties is to reduce the grain size since it may increase the yield stress and promote superplastic forming. Grain refinement can be achieved by conventional extrusion but recently equal channel angular extrusion (ECAE) has been applied successfully in the case of magnesium alloys. This paper focuses on the effect of such thermo-mechanical treatments on the microstructure and resulting mechanical properties at both room and high temperatures of an AZ91 alloy. Depending on the conditions of treatment, the yield stress can reach values close to 400 MPa and elongations to fracture of about 1000% are obtained in the superplastic regime. Nevertheless, the effect of grain refinement on mechanical properties must be considered with care since other strengthening effects may be induced by the processing route.