The nature of electron transfer between the bound complex cytochrome c and cytochrome c peroxidase has been investigated. Experimental verification of the predicted charge-transfer band provides evidence of electron tunneling as the mechanism of transfer between these molecules in solution at room temperature. The measured transfer distance is ≃ 7 angstroms between heme edges, which results in a distance of ≃ 15 to 20 angstroms between iron atoms.