This talk will describe efforts to better understand relationships between the structure of a protein film immobilized on an electrode and its electrochemical activity, which is a prerequisite to design of protein-based devices in which vectorial, heterogeneous electron transfer is required for efficient operation. The relationship between molecular orientation and electron transfer in immobilized films of cytochrome c adsorbed to an indium-tin oxide (ITO) electrode is being investigated by using a novel form of electroreflectance spectroscopy, potential-modulated attenuated total reflection spectroscopy (PM-ATR). Only about half of the protein film is electroactive, which makes it difficult to correlate the broad orientation distribution (measured spectroscopically on the entire film) with the electron transfer rate (measured electrochemically on the electroactive portion). PM-ATR provides a unique approach to this problem. In this technique, the output of a planar waveguide electrode is monitored while an ac potential modulation is simultaneously applied. Changes in the absorbance as a function of the light polarization, modulation frequency, and amplitude provide information about electron transfer rates, electro-optical switching rates, and molecular orientation. For cytochrome c films on ITO, the electron transfer rate measured using TM polarized light was four-fold greater than that measured using TE polarized light, which is consistent with a shorter tunneling distance for molecules adsorbed in a vertical orientation (probed with TM) vs. molecules adsorbed in a horizontal orientation (probed with TE). These data are the first to correlate a distribution of molecular orientations with a distribution of electron transfer rates in a redox-active molecular film.