Benzoquinone/hydroquinone redox inter conversion by the reversible Os(dmb)(3)(3+/2+) couple over an extended pH range with added acids and bases. has revealed the existence of seven discrete pathways. Application of spectrophotometric monitoring with stopped-flow mixing has been used to explore the role of PCET. The results have revealed a role for phosphoric acid and acetate as proton donor and acceptor in the concerted electron-proton transfer reduction of benzoquinone and oxidation of hydroquinone, respectively.