Thin film voltammetry was used to obtain direct, reversible, electron-transfer peaks between electrodes and the spinach photosystem II (PS II) reaction center in lipid films for the first time. Three well-defined pairs of reduction-oxidation peaks were found using cyclic and square wave voltammetry at 4 degrees C at pH 7.5, reflecting direct, reversible electron transfer involving cofactors of PS II. These peaks were assigned to the oxygen-evolving complex (OEC) tetramanganese cluster (E-m = 0.2 V vs NHE), quinones (E-m = -0.29 V), and pheophytin (E-m = -0.72 V). PS II that was depleted of the OEC did not give the peak at 0.2 V. Observed E-m values, especially for the OEC, may be influenced by protein-lipid interactions and electrode double-layer effects. Voltammetry at pH 6 and at pH 7.5 with a time window of > 100 ms revealed that the manganese cluster oxidation is gated by slow deprotonation of a reduced form. Additional rapid protonation/deprotonation steps are also involved in the electrochemical reduction-oxidation pathways.