This paper reports an in situ XRD study on the multisubstituted intermetallic compound MmNi3.85Mn0.27Al0.37Co0.38 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<x≤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 β′ phase compared with the fully charged β phase.