A series of Co-S alloys were synthesized by means of ball milling of Co and S powders at different hours and investigated as the negative material for Ni/MH batteries. The structures and surface configuration of the alloys were characterized by XRD and TEM. The electrochemical measurements demonstrated that the Co-S particles showed excellent electrochemical reversibility and considerably high charge-discharge capacity. Among the alloys, the Co-S alloy milled 20 h showed relatively high discharge capacity and excellent cycling stability at discharge current density 25 mA/g. Its highest discharge capacity was about 350 mAh/g and remained 300 mAh/g after 100 cycles, the capacity retention rate was about 86%. The hydrogen storage mechanism was studied by XRD and TPD measurements. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.