Literature data on reversibility of gas, metal-oxide and membrane ZrO2(Y2O3 or CaO) oxygen electrodes have been considered together with the obtained data for molten alkaline halogenide melts. An explanation of the existence of two slopes for Pt-based electrodes has been proposed. A metal-oxide and a membrane electrodes have been used for studies of dissociation of some oxide-ion donors in molten alkaline halogenides in order to check the completeness of dissociation. NaOH and BaO have been found to be completely dissociated bases in molten chlorides, CO32- ion was a strong, but incompletely dissociated base. Dissociation of BaO became incomplete in molten bromides.