A procedure was developed by which manganese dioxide electrodes can be reproduced within ±0.005 volt with manganese dioxide prepared by precipitation or by thermal decomposition of manganese (II) nitrate. Carbon mixed with either type of manganese dioxide lowers the electrode potential when exposed to electrolyte. The reaction involved is probably electrochemical; it takes place with electrical connection between dioxide and carbon instead of actual contact if these are in contact with the same electrolyte. The reaction is explained as the release of oxygen at the carbon surface by the oxidizing action of the manganese dioxide. This explanation was tested by measurement of the oxygen concentration in a solution originally oxygen‐free, but in contact for some time with manganese dioxide and carbon.