The composite system “Ca(BH4)2-Mg2NiH4” is the first reported that has an effective boron transfer between the compounds of the mixture, which makes it ideally suited for a reversible hydrogen storage system.Using the Kalpha/Kbeta ratio from muonic X-rays emitted by boron, which is sensitive to the local environment, we would like to identify which phases are present in the system upon desorption of hydrogen and when H2-absorption triggers the return of boron in the native Ca(BH4)2 framework.To obtain this achievement, we will use high-resolution measurements of X-ray emission from boron and such results are expected to have a pioneering nature in making Muonic X-ray technique a local probe comparable to X-ray emission spectroscopy.