The use of Mg as hydrogen storage material is hampered by slow sorption kinetics and high thermodynamic stability. This work reports on biphasic Mg–Ti–H nanoparticles that outperform known Mg-based materials in both respects. By exploiting gas-phase condensation of Mg and Ti vapors under He/H 2 atmosphere, biphasic nanoparticles are grown, in which the bulk-immiscible MgH 2 and TiH 2 phases are mixed at the nanoscale. TiH 2 conveys catalytic activity for H 2 dissociation/recombination and accelerated hydrogen diffusion, while MgH 2 provides reversible hydrogen storage. At the remarkably low temperature of 150° C, hydrogen absorption and desorption are completed in less than 100 s and 1000 s, respectively. Moreover, the equilibrium pressure for hydrogen sorption exhibits a composition-dependent upward shift compared to bulk Mg, resulting in a pressure increase by a factor of about 4.5 in the Ti …