Uniform mesoporous beta-Ni(OH)(2) and NiO nanospheres with hierarchical structures were synthesized by a facile complexation-precipitation method. The effects of ammonia and citrate on the structure and morphology of the products were thoroughly investigated by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption-desorption measurements. The results indicated that ammonia played an important role for the formation of flowerlike spheres assembled from nanosheets. The addition of citrate could remarkably reduce the particle sizes and increase the specific surface areas of flowerlike spheres. A possible formation mechanism based on the experimental results was proposed to understand their growing procedures. beta-Ni(OH)(2) and NiO nanospheres prepared with the addition of citrate showed excellent capacitive properties due to their mesoporous structures and large surface areas, suggesting the importance of controlled synthesis of hierarchical nanostructures for their applications. (C) 2012 Elsevier Ltd. All rights reserved.