Transmembrane electron transfer across phospholipid bilayer membranes (BLMs) mediated by ubiquinone have been investigated. The membranes were prepared by the brush method and patch-clamp micropipet techniques and were used to separate identical aqueous Fe(II)/Fe(III) citrate solution. Electron transfer across the BLMs was observed on applying a potential difference across the system, The current, and thus the transmembrane electron transfer rate, were found to be dependent on the redox potential of the bathing solutions. The overall transfer coefficient was 0.5, and it was concluded that the rate-determining step is the transfer of the semiubiquinone radical anion across the hydrophobic interior of the membrane, instead of charge transfer reactions at the membrane/electrolyte solution boundaries. The results obtained could not be understood in terms of tunneling to the neutral ubiquinone molecule residing in the bilayer.