Electrochemical redox reactions of electrochemically precipitated thin films of nickel hydroxide have been studied by an electrogravimetric method with the aid of electrochemical quartz crystal microbalance. Cyclic voltammograms show that in alkaline solutions alpha-Ni(OH)(2) gains mass during its oxidation and loses mass during reduction. The mass change of alpha-Ni(OH)(2) film is explained by the process of intercalation and deintercalation involving K+ and H+ ions. beta-Ni(OH)(2) film, on the other hand, shows a mass change behavior completely opposite to that of alpha-Ni(OH)(2), losing mass during oxidation and gaining mass during reduction. Its mass change is explained by the intercalation and deintercalation of H+ ions. Phase transformation from alpha-Ni(OH)(2) to beta-Ni(OH)(2) is found to proceed with a formation of intermediate phases. The cyclic voltammograms taken during the phase transformation show that the intermediate phases behave as single phases. Cyclic voltammograms taken simultaneously, however, show that the intermediate phases behave as a microscopic mixture of alpha-Ni(OH)(2) and beta-Ni(OH)(2).