The electron-tunneling pathway between an electrode and the protein cytochrome c was studied on mixed C15-carboxylic acid/hydroxyl-terminated SAMs and C16-pyridine/alkane-terminated SAMs as a function of the surface concentration and the length of the diluent alkanethiol. It was found that by increasing the concentration of shorter-chain hydroxyl diluent thiol in the SAM the rate constant increased in a systematic way. When the diluent composition is high, the k 0 increases exponentially with decreasing chain length of the diluent molecules until the chain length of the diluent molecule reaches a length of about eight methylene groups. These results indicate that short-chain diluent molecules contribute to the electronic coupling between the protein and electrode, and that shortening of the diluent decreases the effective tunneling distance.