For the first time, Mg based Mg-Transition metal (TM) -La (TM = Ti, Fe, Ni) ternary composite powders were prepared directly through arc plasma evaporation of Mg-TM-La precursor mixtures followed by passivation in air. The composition, phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. Composition analyses revealed a reduction in TM and La contents for all powders when compared with the compositions of their precursors. It is observed that the composites are all mainly composed of ultrafine Mg covered by nano La2O3 introduced during passivation. Based on the Pressure Composition Temperature measurements, the hydrogenation enthalpies of Mg are determined to be -68.7 kJ/mol H-2 for Mg-Ti-La powder, -72.9 kj/mol H-2 for Mg-Fe-La powder and -82.1 kJ/mol H-2 for Mg-Ni-La powder. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the hydrogenated ternary Mg-TM-La composites when compared to those in the binary Mg-TM or Mg-RE composites. This is especially true for the Mg-Ni-La composite powder, which can absorb 1.5 wt% of hydrogen at 303 K after 3.5 h. Such rapid absorption kinetics at low temperatures can be attributed to the catalytic effects from both Mg2Ni and La2O3. The results gathered in this study showed that simultaneous addition of 3d transition metals and 4f rare earth metals to Mg through the arc plasma method can effectively alter both the thermodynamic and kinetic properties of Mg ultrafine powders for hydrogen storage. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.