The performance of selected LaNi5-yAly hydride electrodes was studied by extensive electrochemical measurements and in situ neutron-diffraction measurements of the deuterated electrode (MDx) during electrochemical charge-discharge cycles. A small addition of aluminum increased the capacity tenfold under ambient conditions. Increased cell impedance and reduced capacity were noted through the cycle life of LaNi5-yAlyHx/NiO(OH) cells and were found to be associated with the corrosion and leaching of aluminum from the alloy. A high aluminum content alloy (y = 0.6), however, compensated for the corrosion loss of aluminum by achieving a longer cycle life than that of a low-aluminum-content alloy (y = 0.12). In situ neutron diffraction indicated that only alpha phase was present in the low-aluminum-content alloy, LaNi4.88Al0.12D1.1, while both alpha and beta phases were present in LaNi4.4Al0.6D1.8, the fully charged state. With in situ neutron-diffraction measurements, the absolute values of x in the MDx formula can be determined for various charge/discharge states, while the coulometry of cell cycling measures only the change in x.