On the basis of the crystal phase evolution from beta-Ni(OH)(2) to alpha-Ni(OH)(2), alpha-Ni(OH)(2), [3-Ni(OH)(2), and their mixed-phase structures were selectively crystallized at different stages during the hydrolysis of Ni(Ac)(2) at hydrothermal conditions. We used both XRD characterization and SEM observation to clearly identify such a transformation process. Electrochemical tests indicate that the mixed-phase Ni(OH)(2) obtained at 3 h possesses the best capacitive performance among all as-obtained samples such as initially formed beta-Ni(OH)(2) nanosheet aggregates, mixed-phase Ni(OH)(2) obtained at other reaction stages, and the ultimately formed alpha-Ni(OH)(2) nanobelts. This phase selective route provides us an effective way to investigate the correlation between crystal phase composition of electrode materials and their capacitive performance. (C) 2012 Elsevier Ltd. All rights reserved.