A membrane-associated enzyme, cytochrome c oxidase, has been put in lipid bilayer membranes attached to gold electrode surfaces. Gold/thiol self-assembly chemistry and deoxycholate dialysis procedures have been used to insert cytochrome c oxidase into a lipid bilayer on gold surfaces with a controlled orientation. Voltammetric and spectroelectrochemical results indicate that direct electron-transfer communication between the gold electrode surface and cytochrome c oxidase has been achieved. Moreover, immobilized cytochrome c oxidase can both reduce and oxidize solution-resident cytochrome c by appropriately controlling the applied electrode potential.