Nickel metal hydride (NiMH) batteries are used in wireless electric tools and hybrid electric cars recently, which produces many spent NiMH batteries. Reuse and recycle of them is a must to eliminate possible pollutions, as well as to make sustainable production of them, as they use quite a lot of nickel, cobalt and rare earths. Here we report the separation of cobalt from Ni(OH)(2) of spent NiMH batteries, which is required by a recycle process proposed by us, with a reduction and dissolution process in alkali solution. It shows that the alkali concentration plays a key role in the separation of cobalt from spent positive electrode materials. When the concentration of KOH solution increases from 1.0 mol L-1 to 11.0 mol L-1, the cobalt content in the solid materials continuously decreases from 5.09 wt% to the minimum of 1.84 wt% because of the reduction of the CoOOH (and/or Co3O4) formed during the charging of electrodes and dissolution of Co(OH)(2) in the alkali solution. When the concentration goes further higher, however, it increases a little. The regenerated Ni(OH)(2) remains the structure of beta-Ni(OH)(2). In addition, cobalt in the filtrate can be recovered by air oxidation and be reduced into Co(OH)2 by hydrazine, which can be again used as the conductive additive for Ni(OH)(2) positive electrodes. (C) 2013 Elsevier B.V. All rights reserved.