MgH2-M (M = MWCNTs or TiF3 or both of them) composites prepared by high-energy ball milling (HEBM) are used in this work to illustrate the effect of catalysts on isothermal activation, thermodynamic and hysteresis of MgH2 hydrides. The phase compositions, microstructures, particle morphologies and distributions of MgH2 with catalysts have been evaluated. The isothermal synergetic catalytic-activation and dehydrogenation effect of MWCNTs and TiF3 evaluated by P-C-T give the evidences that the addition of catalysts is an effective strategy to destabilize MgH2 and reduce the dehydrogenation temperatures. The isothermal activation process can be remarkly accelerated by adding MWCNTs or TiF3. Its worthnoting that fast initial absorption rate and high hydrogenation capacity are obtained for modified MgH2 when adding MWCNTs coupling with TiF3 and no special activation treatment is needed. The activation effect is mainly attributed to the large contraction/expansion stresses caused by accelerated catalytic desorption/absorption cycles. The catalytic effect on thermodynamic is mainly attributed to electronic exchange reactions with hydrogen molecules during the dissociation-absorption or recombination-desorption process. The thermodynamic hysteresis of catalyzed MgH2 is also investigated to evaluate the energy consumption and estimate the efficiency of the isothermal hydrogen absorption/desorption process. A probable synergetic catalytic-activation mechanism is probed. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.