Phase transformation of electrochemically precipitated nickel hydroxide has been studied by the electrogravimetric method using an electrochemical quartz crystal microbalance. Cyclic voltamassograms show that alpha-Ni(OH)(2) gains mass during its oxidation and loses mass during reduction in alkaline solutions. 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. alpha-Ni(OH)(2) is unstable in alkaline solutions and thus undergoes a phase transformation to beta-Ni(OH)(2) with a formation of intermediate phases. Cyclic voltammograms taken during the phase transformation show that the intermediate phases behave, electrochemically, as a single phase. Cyclic voltamassograms taken simultaneously, however, show that the intermediate phases behave as a microscopic mixture of alpha-Ni(OH)(2) and beta-Ni(OH)(2). Cyclic voltamassograms of nickel hydroxide films with coprecipitated Co show more clearly the existence of the intermediate phases which behave as the. microscopic mixture of alpha-Ni(OH)(2) and beta-Ni(OH)(2) in mass change. The measured cyclic voltamassograms are compared with the simulated voltamassograms obtained from the present phase transformation model. The comparison shows that the present model incorporating the concept of the microscopic mixture of alpha-Ni(OH)(2) and beta-Ni(OH)(2) explains successfully the gradual reversal of mass change behavior during the phase transformation.