The goal of the research was to develop a transmission spectroelectrochemical method with an optically transparent boron-doped diamond electrode that is useful for studying structure-function relationships of redox proteins. First, the electrochemical behavior of the cytochrome c was investigated at boron-doped ultrananocrystalline diamond (UNCD) and microcrystalline diamond (MCD) electrodes. Quasi-reversible, diffusion-controlled electron transfer was observed at oxygen-terminated, but not hydrogen-terminated electrodes. UNCD electrodes exhibited a similar electrochemical response as the MCD electrodes but showed higher peak currents due to a larger electrochemical active area. pH-dependent studies of the voltammetric response revealed that the largest peak current and smallest peak separation were observed between pH 7 and 7.5. Next, a custom-designed thin-layer transmission spectroelectrochemical cell was constructed and characterized. Electrochemical characterization of the thin-layer cell showed Gaussian-shaped thin-layer voltammetric i-E curves with a linear relationship between the peak current and the scan rate. Time-dependent measurements indicated rapid and complete electrolysis in the cell, and good agreement between electrochemical and spectral data. The spectroelectrochemical studies of cytochrome c revealed an advantage of diamond OTE in terms of its fouling resistance. The spectral characterization of the cytochrome c-cyanide complex was also presented for the first time. The reversible spectroelectrochemical response of myoglobin demonstrated the applicability of the cell under anaerobic condition. An analysis method was developed for studying a multicomponent redox protein, Na+-translocating NADH:quinone oxidoreductase (Na +-NQR). The UV-Vis difference spectra of this protein at diamond OTE without mediators indicated that the reversible reaction and complete electrolysis occurred in the thin-layer cell. The spectra were modeled with global regression analysis to resolve the redox centers in Na+-NQR and determine the corresponding formal potentials.