Is this paper, the electrodeposition/electrodissolution cycling of lead dioxide (PbO2) is studied on vitreous carbon electrodes in lead methanesulfonate/methanesulfonic acid medium for a soluble lead acid flow battery application. The influence of the active species concentrations (Pb2+, H+) on the cyclability of lead dioxide (charge efficiency, lifetime of cycling, etc.) is assessed. The proton has a very adverse effect when its concentration is above 1M. Electrochemical impedance spectroscopy (EIS) shows that the lead dioxide layer can be passivated at the end of reduction when the electrolyte is strongly acidic. Mass changes investigations carried out during PbO2 cycling with a quartz crystal microbalance (QCM) confirm the presence of the solid-state reaction predicted in the literature. This side reaction suggests the formation of a non-stoichiometric PbO x compound with a poor solubility and a high electric resistance. The effect of the acid concentration as well as the current density on the formation of PbO x is assessed. The poor cyclability of PbO2 in high acidic media can be related to the accumulation of this resistive compound within the layer. Finally, considerations for an optimal operation of the battery are presented.