The oxidation of plastoquinol by the cytochrome bf complex is commonly believed to be the rate limiting step in photosynthetic electron transport. When input of electrons from PS II exceeds electron flow through the cytochrome bf complex the plastoquinone pool becomes reduced. A voltammetric technique previously used to measure the redox state of the ubiquinone pool in plant mitochondria, was modified to measure the redox state of the plastoquinone pool in thylakoids. The presence or absence of a proton gradient strongly influenced the relationship between the redox state of the plastoquinone pool and other photosynthetic parameters. A linear relationship between the rate of electron transport and the reduction of plastoquinone was found. The slope of this relationship was greater in coupled than in uncoupled thylakoids, indicating that under coupled conditions the plastoquinone pool is more reduced at any given rate of electron flow. A complex relationship was found between QA reduction, calculated as 1 – q_p, and the redox state of the plastoquinone pool. The extent of Q_A reduction was similar in coupled and uncoupled thylakoids, but at any given level of Q_A reduction, PQ was always more reduced in coupled thylakoids. These results suggest that the presence of a proton gradient changes the equilibrium constant between Q_A and PQ.