Ternary Mg1-xCaxNi2-y solid solutions were synthesized by powder sintering. The phase structures and hydrogen storage properties of the sintered samples were investigated. In a certain range of x and y values, the samples are a single C15 Laves phase with various types of defects. The reduction of Ni content leads to the formation of omission solid solution with vacancies on the sites of Ni. These vacancies increase the hydrogen storage capacity, but decrease the reversibility of hydrogen absorption and desorption. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.