Abstract The microstructures, thermal stability and high deformation behavior of ultrafine grained (UFG) AZ61 alloys with trace amounts of Ti (0.01–0.02wt.%), which were prepared by extrusion followed by high-ratio differential speed rolling (HRDSR), were examined. The addition of trace amounts of Ti significantly altered the morphologies and amounts of β (Mg17Al12) phase in the alloys. During extrusion, most of β phase dissolved into the matrix, forming a unique microstructure with a number of the Al-rich strip-like bands elongated to the extrusion direction. During subsequent HRDSR, very fine β phase particles were precipitated from the Al-rich band structures and distributed uniformly on the UFG matrix. The addition of Ti also greatly affected the high temperature deformation behavior and superplasticity of the UFG AZ61 alloy by reducing its thermal stability through rapid dissolution of β-Mg17Al12 particles into the matrix at temperatures⩾523K. The strain rate-stress relations for the AZ61 alloys with and without trace amounts of Ti, having a wide range of grain sizes between 3 and 145μm, could be well described using a single constitutive equation, assuming that Coble creep, grain boundary sliding, solute drag creep and dislocation climb creep compete with one another.