The magnitude of previously reported pH changes at the surface of Chara corallina cells has been measured with specially constructed pH electrodes. An experimental system was designed to reduce the dissipation of pH gradients by convection. When cells are illuminated their surfaces become partitioned into alkaline regions (pH 8.5–9.5) with sharp peaks and acid regions (pH about 5.5) which are more uniform. In the dark these regions rapidly disappear and the pH of the cell surface becomes uniform (about pH 6.0). With cells initially free of any deposits of CaCO3, alkaline regions may appear temporarily, and may migrate along the cell surface. The presence of CaCO3 stabilizes the regions. It is proposed that alkalinization results from HCO3− uptake, CO2 fixation, and subsequent localized OH- efflux. Alkalinization is prevented by photosynthetic inhibitors. The nature and magnitude of the HCO3−/OH- exchange process is discussed, and the possible role of H+ and OH- fluxes in controlling the ionic and electrical properties of Characean cells is emphasized.