In this paper Co-doped Al-substituted alpha-Ni(OH)(2) was prepared by a homogeneous precipitation method and its electrochemical behavior, in two kinds of electrolyte (7 M KOH solution and 7 M KOH saturated with Al(OH)(3)), was investigated by galvanostatic charge-discharge method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical properties of the commercial beta-Ni(OH)(2) in both electrolyte was also studied in order to compare with that of alpha-Ni(OH)(2). The results show that Co-doped Al-substituted alpha-Ni(OH)(2) has the largest capacity in 7 M KOH solution (300 mAh g(-1)). The addition of Al to KOH solution is harmful to the reversible capacity and cycle life of both alpha-Ni(OH)(2) and beta-Ni(OH)(2). X-ray diffraction (XRD) patterns show that, for Co-doped Al-substituted alpha-Ni(OH)(2), its turbostratic structure maintains after cycles in both alkaline electrolyte, despite the interlayer distance becoming smaller. For beta-Ni(OH)(2), however, gamma-NiOOH species appeared after cycling in both electrolyte and its content is higher after cycling in 7 M KOH saturated with AI(OH)(3). And adding Al into KOH solution caused the decrease of solution conductivity and increased the apparent diffusion coefficient which is evidenced by electrochemical impedance spectroscopy. (C) 2004 Published by Elsevier B.V.