Abstract An electrochemical thin-layer cell, 0.5 ml in volume and 0.25 mm in pathlength, employing an optically transparent gold minigrid electrode has been constructed and used in redox studies of photosynthetic electron-transport components. Light-induced absorption changes accompanying P700 photo-oxidation in photosystem-I subchloroplasts were measured at a series of electrochemically poised potentials at 88°K. The fluorescence-yield changes in photosystem-II subchloroplasts were measured as the poised potential was varied electrochemically. The utility of the cell was further demonstrated by a redox titration of soluble (spinach) ferredoxin using circular dichroism to monitor changes in the redox state. One important advantage of electrochemical vs chemical titration of photosynthetic electron-transport components is that it is possible to attain much more negative potentials than is thermodynamically possible in chemical titrations. This permits the exhaustive titration of certain components at physiologically reasonable pH values not possible chemically.