A ZrV0.5Mn0.5Ni alloy negative electrode was modified by immersing it in an alkaline solution containing various concentrations of KBH4 as a reducing agent. At 40 degrees C the immersion of the electrode in alkaline 0.01 M KBH4 solution for 24 h gave rapid activation and the maximum hydrogen storage capacity. It was found from X-ray diffractometry that the lattice volume was expanded due to the absorption of hydrogen atoms released from BH4- into the alloy, resulting in the pulverization of the alloy particles and the formation of the new electroactive surfaces. This fact was further confirmed from the SEM image of the alloy and the change of potential of the alloy electrodes during the course of the immersion in alkaline KBH4 solution. The immersion temperature significantly affected the activation and the discharge capacity of the alloy electrodes.