The effect of partial substitution of Co for Ni on the structure and electrochemical properties of the thus formed La2Mg(Ni1-xCox)(9) (x = 0.1-0.5) quaternary alloys was investigated. All alloys are consisted of a main phase with hexagonal PuNi3-type structure and a few impurity phases (mainly La2Ni7 and LaNi). The increase of Co content in the alloys leads to an increase in both the cell volume and the hydride stability, and leads to a noticeable decrease in cell volume expansion rate (Delta V/V on hydriding. The discharge capacity of the alloys at 50 mA/g increases slightly with the increase of Co content and passes though a maximum of 404.5 mAh/9 at x = 0.2. As the Co content increases, the high-rate dischargeability of the alloy electrodes at 800 mA/g (HRD800) decreases sharply from 72.8 (x = 0) to 24.5 % (x = 0.5), yet the decrease of HRD800 of the alloy electrodes with lower Co substitution (with x less than or equal to 0.2) is much milder. The slower decrease of HRD800 (from 72.8 to 64.2%) of the alloys with x from 0 to 0.2 is mainly attributed to the decrease of eletrocatalytic activity for charge-transfer reaction, the more rapid decrease of the alloys with x > 0.2 is mainly attributed to the lowering of the hydrogen diffusion rate in the bulk of alloy. The cycling capacity retention rate (S-100) of the alloys increase greatly with increasing of Co content, increasing from 60.2% for the alloy with x = 0 to a much higher value of 87.9% for the alloy with x = 0.5. The improvement in cycling stability is attributed to the lower cell volume expansion on hydriding. (C) 2004 Elsevier Ltd. All rights reserved.