The potential of the reaction ▪ and the chemical equilibrium ▪ have been studied for compositions near to MnO 2 where the oxyhydroxide is a single homogeneous phase. An electrodeposited γ-MnO 2 and a Caucasian β-MnO 2 ore have been used as starting materials and compositions of reduced stoichiometry have been reached by means of the chemical equilibrium shown above. The concentration of MnOOH is insufficient to affect significantly the chemical potential of MnO 2 and potential and chemical equilibrium are controlled by pH and the activity of MnOOH. The behaviour observed can be quantitatively understood on the basis that in the β-MnO 2, MnOOH is undissociated so that its activity is equal to its mole fraction but that in the γ-MnO 2, MnOOH is dissociated so that Mn 3+ and OH − have independent existences and its activity is related to the product of the mole fractions of these ionic constituents.