MgCo_2 and MgNiCo crystallize with hexagonal Laves type intermetallic structures of the C14 type and do not form hydrides at ambient hydrogen pressures. However, applying high hydrogen pressures in the GPa range forces the hydrogen absorption and leads to the formation of multi-phase compositions, which contain approximately 2.5 atoms H per formula unit of MgCo_2 or MgNiCo and remain thermally stable under normal conditions.The hydrogenation of MgCo_2 resulted in its decomposition to a ternary Mg_2CoD_5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27–500 °C were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo_2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300°C, the Mg_2CoD_5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400°C, a MgCo phase was formed by interaction between Mg and Co. At the highest processing temperature of 500°C, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo_2.The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8 GPa and 200 °C proceeded in a more complex way. A very stable ternary deuteride MgNi_2D_3 was leached away while Co was separated in the form of Mg_2CoD_5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni_(0.7)Co_(0.3).The thermal desorption of deuterium from MgCo_2D_(2.5) and from MgNiCoD_(2.5) has been studied by Thermal Desorption Spectroscopy with deuterium released into a closed volume. The observed effects nicely correlate with changes in the phase structural composition of the hydrides formed.MgCo_2 is a new example of the hydrogen storage alloy, in which a successful HDDR processing results in the reversible formation of the initial intermetallic at much lower temperatures than in the equilibrium phase diagram of the Mg-Co system. MgCo_2 and MgNiCo crystallize with hexagonal Laves type intermetallic structures of the C14 type and do not form hydrides at ambient hydrogen pressures. However, applying high hydrogen pressures in the GPa range forces the hydrogen absorption and leads to the formation of multi-phase compositions, which containing-2.5% H per formula unit of MgCo_2 or MgNiCo and remain thermally stable under normal conditions. The hydrogenation of MgCo_2 resulted in its decomposition to a ternary Mg_2CoD_5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27-500 ° C were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo_2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300 ° C, the Mg_2CoD_5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400 ° C, a MgCo phase was formed by inte raction between Mg and Co. At the highest processing temperature of 500 ° C, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo_2. The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8 GPa and 200 ° A very stable ternary deuteride MgNi_2D_3 was leached away while Co was separated in the form of Mg_2CoD_5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni_ (0.7) Co_ (0.3). thermal desorption of deuterium from MgCo_2D_ (2.5) and from MgNiCoD_ (2.5) has been studied by Thermal Desorption Spectroscopy with deuterium released into a closed volume. The observed effects nicely correlate with changes in the phase structural composition of the hydrides formed. MgCo_2 is a new example of the hydrogen storage alloy, in which a successful HDDR processing results in the reversible formation of the initial intermetal at much lower temperatures than in the equilibrium phasediagram of the Mg-Co system.