The effect of selected transition metal fluorides on the sorption properties of Ca(BH4)2 and Ca(BH4)2 + MgH2 systems is investigated. The (re)hydrogenation kinetic is improved by the addition of transition metal fluorides. The formation of transition metal boride nanoparticles is observed. These nanoparticles are found to be essential to promote the heterogeneous nucleation of CaB6 during hydrogen desorption. Under the experimental conditions reported in this study, the formation of CaB12H12 hinders the reversibility. In case of the Ca(BH4)2 + MgH2 composite system, the (re)hydrogenation reaction is promoted without additives. In fact, Mg acts as heterogeneous nucleation agent of CaB6 during decomposition of the Ca(BH4)2 + MgH2 composite system. The role of CaB12H12 over cycling and its effect upon (re)hydrogenation is investigated.