High-pressure synthesis is an effective technique to obtain novel compounds. Under high pressure of the order of GPa, the atomic radius of Mg undergoes large shrinkage compared with that of transition metals (TM) and novel compounds with unusual atomic sizes of Mg and TMs can be synthesized. This study describes the high-pressure synthesis of a novel Mg-Co alloy of formula Mg44Co7 at above 5 GPa and 973 K for 8 h. Investigation of the crystal structure of Mg44Co7 using X-ray powder diffraction reveals its Mg44Rh7-type structure (space group F-43 m) with lattice parameter a = 20.127 (1) angstrom. Mg44Co2 decomposes into Mg and MgCo phases at 663 K in an Ar atmosphere via an exothermic reaction. Hydrogenation of Mg44Co7 at 573 K-623 K under 8 MPa of H-2 results in its decomposition into MgH2, Mg2CoH5 and Mg6Co2H11 with a total hydrogen content of 3.5-5.0 mass%. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.