We have tested the hydrogen storage cycling behavior of bulk centimeter-scale magnesium - titanium and magnesium - stainless steel multilayer composites synthesized by accumulative roll-bonding (ARB). Roll-bonding of either titanium or stainless steel with magnesium allows the reversible hydrogen sorption of the resulting composite at 350 C. Identically roll-bonded pure magnesium can hardly be absorbed at this temperature. In the composites, the kinetics of the first cycle of absorption (also called "activation") improves with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Increasing the loading amount of the second phase also accelerates the kinetics. This holds true up to a threshold limit. Microscopy analysis performed on 1-2 wt.% hydrogen absorbed composites demonstrates that MgH(2) formed exclusively on various heterogeneous nucleation sites. During activation, MgH2 nucleation occurred at the Mg-hard phase interfaces. During the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of "internal" free surfaces such as cracks. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.