The charging voltage of an electrochemical cell consisting of Pb/PbSO, and Hg/Hg,S04 electrodes in aqueous HaSO increased rapidly from a low to a high plateau region. The time required for this transition to occur increases with decreasing acid concentration. This has been attributed to the complete passivation of the mercury anode by an Hg,SO, layer and the subsequent replacement of the existing process (Hg+Hg2S0,) by another one (Hg--r Hg'+) under this layer. Though the discharge efficiency of anodically grown Hg,SO, is found to increase with acid concentration, the discharge capacity of the mercury electrode decreases due to its lower charge acceptance characteristics at higher acid concentration. Nevertheless, the overall discharge efficiency of the total cell is found to pass through a maximum when the acid concentration is varied. On cycling the cell at any particular current density, both the discharge efficiency and the capacity are found to increase over the first few charge-discharge cycles before assuming fairly steady values.