Nickel hydroxide is a well known electrode active material for nickel-based batteries. Compared with a commercial spherical Ni(OH)(2), characteristics of an ultrafine Ni(OH)(2) made from supersonic co-precipitation method were studied. SEM showed that the prepared Ni(OH)(2) exhibited a worm-like structure, compared to the ball-like shape of spherical Ni(OH)(2). By XRD, the sample was found to be P-type Ni(OH)(2), with more structure distortion due to a shortening in Ni-Ni distances and a prolongation in distances between NiO2 layers as the lattice constants for the sample are a = 3.123 angstrom, c = 4.655 angstrom. XANES testified a severe structure distortion for the prepared Ni(OH)(2), and EXAFS showed expansion in Ni-O and contraction in Ni-Ni distances when compared to perfect crystal Ni(OH)(2). Electrochemical impedance spectroscopy (EIS) determined that proton diffusion coefficient of the ultrafine powder (6.0 x 10(-16) cm(2) s(-1)) was about two orders larger than that of spherical Ni(OH)(2) (5.6 x 10(-18) cm(2) s(-1)). The badly crystallized ultrafine Ni(OH)(2) may have more electrochemical activities and thus could be applied in nickel-based batteries. (C) 2006 Elsevier B.V All rights reserved.