The hydrogen sorption properties of the nanocomposite 85 wt.% Mg-15 wt.% Mg2Ni0.8Co0.2 obtained by mechanical alloying in inert atmosphere were investigated. Absorption measurements were performed under a hydrogen pressure P = 1 MPa at temperartures ranging from 373 to 573 K, while desorption studies proceeded at P = 0.15 MPa and temperatures of 573 and 553 K. The addition of the intermetallic compound Mg(2)Ni(0.8)Co0(0.2) was shown to improve the hydriding kinetics of magnesium. The composite exhibited a high hydrogen capacity which did not decrease even after a large number of absorption-desorption cycles. Comparison of the hydriding kinetics of the intermetallic compounds Mg2Ni and Mg2Ni0.8Co0.2 indicated facilitation of the process by the presence of cobalt in the alloy. Magnetic measurement data on Mg2Ni0.8Co0.2 showed formation of superparamagnetic precipitations of nickel and cobalt playing the role of active centres for dissociative chemisorption of hydrogen. The behaviour of the composite was explained by the catalytic effect of the intermetallic Mg(2)Ni(0.8)Co0(0.2,) the existence of Ni and Co clusters on the surface and the process of mechanical alloying.