The effects of Mg content on the crystal structures and hydrogenation behavior of (Ca1-xMgx)Al-2,alloys have been studied. It was found that the (Ca0.9Mg0.1)At, alloy consisted of a major phase C15 and a small amount of CaAl4. Increasing x to 0.2, the C36 phase was present besides the phases C15 and CaAl4. Further increasing x to 0.3, the alloy contained four phases C15, CaAl4, C36 and Al. When x=0.4, the alloy consisted of the C15. C36 and Al phases. In the Laves phases C15 and C36, Mg atoms occupied both the Ca site and the Al site. Thus the lattice parameters of the Laves phases decreased with increasing x. The C15 phase did not react with hydrogen but the C36 phase decomposed into CaH2, and Al during hydrogenation at 473 K. (c) 2007 Elsevier B.V. All rights reserved.