The hydrogenation and dehydrogenation behaviours of the YNi3.5Al0.5Mg compound were studied by in situ X-ray diffraction under hydrogen pressure and at room temperature. The changes of (i) the lattice parameters, (ii) the crystallite size and (iii) the lattice strain during the sorption process (i.e. along the PC isotherms) were studied. These results indicate that the crystallite size decreases by a factor of 2. The micro deformations increase at first and then tend to almost zero at the end of the sorption cycle. This behaviour is explained in terms of co-existence of the metal (i.e. alpha phase) and metal hydride (i.e. beta phase) phases. The change in crystallinity is consistent with the hydrogen induced amorphisation process existing in a lot of AB(2) compounds. No anisotropic effects can be highlighted on this pseudo-AB(2) compounds in contrary with what could be observed in AB(5) compounds. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.