Further measurements are reported for the standard potentials of variously prepared and treated samples of β-manganese dioxide. Values lie in the range 1.231–1.244 V except for a highly crystalline autoclaved sample (1.196 V). A discrepancy in the ion-exchange behaviour of these almost stoichiometric β-MnO2 samples, compared with that of less stoichiometric forms of manganese dioxide, has been traced to a reaction between manganese dioxide and dilute acids which is rate controlled by diffusion in the oxide.