A good correlation has been obtained between the existence of hydrogen reservoirs in solids revealed by a dynamic method of titration and an in-situ X-ray diffraction analysis under H-2. The dynamic method of titration involves hydrogenation at 423 K of 2-methylbuta-1,3-diene (isoprene) under helium how in the absence of gaseous hydrogen and permits to reveal and titrate reactive hydrogen species (H*) that a solid is able to store. Different compounds, such as a CeNi0.5O2.5 mixed oxide and a Cs(1.)6(NH4)(2.)4P(1.7)Mo(11)V(1.1)O(40) polyoxometalate, are found to be hydrogen reservoirs with marked diffusion properties of hydrogen species. The hydrogen reservoir capacity depends on the pretreatment temperature under H-2, which has been shown to correspond to the creation of anionic vacancies in the solid by the loss of H2O. In the conditions in which the incorporation of hydrogen in the solids occurs, X-ray diffraction analysis under H-2 show shifts depending on the treatment temperature and corresponding to a lattice expansion attributed to the insertion of hydrogen species of hydridic nature in the anionic vacancies.