Reversible hydrogen sorption coupled with the MgH2 ↔ Mg phase transformation was achieved in the remarkably low 340–425 K temperature range using MgH2–TiH2 composite nanoparticles obtained by reactive gas-phase condensation of Mg–Ti vapors under He/H2 atmosphere. The equilibrium pressures determined by in situ measurements at low temperature were slightly above those predicted using enthalpy ΔH and entropy ΔS of bulk magnesium. A single van’t Hoff fit over a range extended up to 550 K yields the thermodynamic parameters ΔH = 68.1 ± 0.9 kJ/molH2 and ΔS = 119 ± 2 J/(K·molH2) for hydride decomposition. A desorption rate of 0.18 wt % H2/min was measured at T = 423 K and p(H2) ≈ 1 mbar, i.e., close to equilibrium, without using a Pd catalysts. The nanoparticles displayed a small absorption–desorption pressure hysteresis even at low temperatures. We critically discuss the influence …