Physiological quinones carrying isoprenoid side chains have been compared with homologues lacking the side chain, for their ability to carry electrons and protons from dithionite to ferricyanide, trapped in liposomes. Six differential observations were made: (1) Plastoquinone and ubiquinones, with a side chain of more than two isoprene units, are by far better mediators than their short-chain homologues. Also other benzoquinones lacking a long side chain are poor catalysts, except dimethyl-methylenedioxy-p-benzoquinone, a highly autooxidizable compound. Tocopherol is a good catalyst. (2) Vitamin K-1 and K-2 are poor mediators compared to vitamin K-3. (3) The reaction catalyzed by quinones carrying long isoprenoid side chains has an about three-fold higher activation energy, irrespective of the catalytic efficiency. (4) The reaction catalyzed by quinones lacking a long side chain follows pseudo first-order kinetics, while the reaction with quinones carrying a long side chain is of apparently higher order. (5) The rate with ubiquinone-1 is increasing pH, while with ubiquinone-9 it is decreasing. (6) The reaction mediated by short-chain quinones seems to be satuarated at lower dithionite concentration. We conclude that isoprenoid quinones are able to translocate electrons and protons in lipid membranes, and that the side chain has a strong impact on the mechanism. This and the relevance of the model reaction for electron and proton transport in photosynthesis and respiration is discussed.