Thermal dehydrogenation of Ca(BH4)2 and Ca(BH4)2-2MgH2 composite has been investigated, and the results were compared. The Ca((BH4)2 dehydrogenated in two steps between 325 °C and 500 °C as per the reactions Ca(BH4)2 + CaH2 + 2B + 3H2, and Ca(BH4)2 = 1/3CaB6 + 2/3CaH2 + 10/3H2. The partial dehydrogenation of CaH2 also takes place during the second step dehydrogenation according to the reaction CaH2 + 6B = CaB6 + H2. The completion of second step dehydrogenation requires a temperature of higher than 500 °C. The activation energies corresponding to these steps were found to be 149 ± 8 kJ/mol and 162 ± 10 kJ/mol, respectively. The Ca(BH4)2-2MgH2 composite dehydrogenates in a single step as per the reaction: Ca(BH4)2 + 3MgH2 = CaMg2 + 2B + 7H2 + Mg. The dehydrogenation of Ca(BH4)2-2MgH2 started at 340 °C and completed before 450 °C. The activation energy of Ca(BH4)2-2MgH2 dehydrogenation was found to be 180 ± 8 kJ/mol.