Thermal decomposition of Co3-xNixO4, x = 0.1, 0.15 and 0.2, at oxygen pressures 2.7-20.0 kPa at linearly growing temperature was examined. Decomposition of Co3-xNixO4 was found to be mixed-control diffusion-reaction process. Observed activation energy of studied process strongly depends on the oxygen pressure-dissociation pressure ratio. Comparison of the results obtained with earlier study on electrical conductivity of Co3-xNixO4 showed linear correlation between activation energy of new phase nucleation and activation energy of electrical conductivity. Transport of electrons from oxygen ions to Co3+ ions leading to CoO nuclei formation was found to be one of two rate limiting steps of thermal decomposition of Co3O4