A novel methodology has been developed to determine electron-transfer rate in electrically driven redox reactions. Based on a widely adopted electrical circuit describing faradaic processes in an electrochemical cell, the approach uses a combination of impedance data from optical and electrical measurements that are simultaneously acquired in a spectroelectrochemical experiment. Once the consistency of our methodology was experimentally corroborated, it was put to practice for investigating electron-transfer rate of cytochrome c adsorbates at very low concentrations on an indium tin oxide electrode by using a highly sensitive, single-mode, electro-active, integrated optical waveguide platform. Different surface densities of redox species on the electrode interface and different ionic strengths in the electrolyte solution were studied. Higher surface densities and higher ionic strengths are shown to slow down the electron-transfer process between the redox molecules and the working electrode.