This paper reports an in situ XRD study on the multisubstituted intermetallic compound MmNi(3.85)Mn(0.27)Al(0.37)Co(0.38) during an electrochemical charge-discharge reaction. Structural data are available through the evolution of the lattice parameters as electrochemical hydrogen absorption desorption proceeds within 0 less than or equal to x less than or equal to 4.3 H per formula unit and a satisfactory agreement is obtained with ex situ neutron diffraction data previously reported for the parent compound LaNi3.55Mn0.4Al0.3Co0.75. Preliminary results show the existence during discharge of a less expanded beta' phase compared with the fully charged P phase. (C) 2003 Elsevier Ltd. All rights reserved.