Hydrogen back pressure remarkably promotes the formation of metal boride during, the dehydrogenation of 4LiBH(4) + YH(3), 6LiBH(4) + CeH(2) and 6LiBH(4) + CaH(2) composites, which seems to be a general phenomenon in LiBH(4)-based reactive, hydride composites that enables mutual destabilization,between LiBH(4) and metal,hydride. The formation of metal boride plays a crucial role in the reversible hydrogen storage properties of these composites. The dependence of the dehydrogenation behavior on hydrogen back pressure might be associated with the microstructural evolution of the dehydrogenation products formed by a solid liquid reaction.