A quartz UV-visible electrolytic cell has been used to study, in situ, formation and consumption of the coloured Mn(OH)63-intermediate which is formed during the processes of discharge and recharge of three types of MnO2 battery cathode materials, including one chemically modified (CM) by Bi(III). The roles of Bi(III) species as dopant in the CM material have been investigated. Firstly, the presence of Bi(III) favours the formation of soluble Mn(OH)63- species in the processes of both discharge and recharge and, secondly, it appears to work as a "catalyst" for the steps involved in further reduction of Mn(OH)63- species to Mn(OH)2 in discharge and reoxidation of Mn(OH)2 to MnO2 on recharge. Based on the absorbance measurements, a heterogeneous mechanism in which the soluble Mn(OH)63- intermediate is involved is directly indicated as being the preferred pathway at CM MnO2 and operates in parallel with the so-called homogeneous mechanism in the processes of both reduction of MnO2 and re-oxidation of reduced MnO2 materials. Thus, from the soluble Mn(OH)63- species, either Mn(OH)2 can be deposited on reduction, or MnO2 re-formed on reoxidation, on the porous electrode matrix at favourable and practically significant current densities. The CM MnO2 is then rechargeable over a practically useable voltage range and over the two-electron charge capacity. On account of the preferred heterogeneous pathway at CM MnO2, high (up to 6C) discharge rates can be realized at a remarkably constant plateau voltage.