A comparison in the hydrogen sorption properties of mechanically activated magnesium composites with additive of 3d-metal containing compounds (Mg(2)Ni(0.8)Co(0.2) and NiCo(2)O(4)) and activated carbon derived from agricultural by-products (ACAS) is accomplished. The presence of the additives used enhances significantly the hydriding reaction and increases the absorption capacity compared to pure magnesium. The absorption capacity values of the composites studied are as follows: Mg (-44 mu m)5 wt.% ACAS-6.13 wt.%, Mg (-297 mu m)-5 wt.% ACAS-5.48 wt.%, Mg(-44 mu m)-15 wt.% Mg(2)Ni(0.8)Co(0.2)-4.78 wt.% and Mg (-44 mu m)-5 wt.% NiCo(2)O(4)-5.69 wt.% (T = 573 K, P = 1 MPa, after 60 min of hydriding). The composites preserve their hydrogen sorption capacities during prolonged cycling. The use of the additive of activated carbon instead of 3d-metal containing compounds in magnesium based composites results, on the one hand, in a higher absorption capacity which increases during cycling, but on the other hand, a poorer hydriding kinetics. (C) 2011 Elsevier Ltd. All rights reserved.