In situ potentiometry and null ellipsometry was combined and, used as a tool to follow the kinetics of biofouling of ion-selective electrodes (ISEs). The study was performed using custom-made solid-contact Ki(+)-ISEs consisting of a gold surface with immobilized 6-(ferrocenyl)-hexanethiol as ion-to-electron transducer that was coated with a potassium-selective plasticized polymer membrane. The,electrode potential and the ellipsometric signal (corresponding to the amount of adsorbed protein) Were recorded simultaneously during adsorption of bovine serum albumin (BSA) at the surface of the K+-ISEs. This in situ method may become useful in developing sensors with minimized biofouling.