The influence on the decomposition and reforming of the hydrogen storage material NaAlH₄ by adding relatively low amounts of mesoporous carbon black is investigated with in situ diffraction. A 60:40 NaAlH₄/carbon black composite is prepared via ball milling and characterised ex situ via X-ray diffraction, gas adsorption, temperature-programmed decomposition, and dehydrogenation/hydrogenation cycling methods. The prepared composite is deuterated, and the crystalline phase composition is determined with in situ neutron powder diffraction method during multiple decomposition/deuteration cycles. Changes in the crystalline phase composition start slightly below the melting temperature of the pristine alanate, whereas the release of deuterium starts at considerably lower temperatures. The decomposition of Na₃AlD₆ to NaD is almost completely reversible at the applied low deuterium pressures of >= 2 MPa. Thus, the strong effect of even low concen-trations of a mesoporous carbon black on the capability to store H₂ reversibly is showcased and analysed in-depth.