The catalytic effect of titanium on the hydrogen sorption properties of a MgYTi alloy has been inves- tigated. The alloy is formed by a majority phase Mg24+x Y5 , a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH2 and YH3 . After hydrogen desorption pure Mg and YH2 were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH2 decomposition enthalpy was determined to −68 kJ/mol H , and the calculated activation energy of hydrogen desorption of MgH2 高壓儲氫研究分析儀（器）