The activity of solid catalysts is often substantially reduced by the formation of coke. The in situ monitoring of the coke load of catalyst particles could therefore be a controlling instrument of high value. Such sensors could be useful to measure the axial coke profile in a fixed bed reactor, both during coke formation and regeneration by coke burn-off. As a contribution to the development of such sensors, single particles of an alumina oxide catalyst were used as a model system. The particles were electrically contacted and characterized by electrical impedance spectroscopy. The coke formation/burn-off and the impedance were simultaneously measured in a magnetic suspension balance. A clear relationship between the coke loading and the respective electrical impedance signal could be observed, both during coke formation and during the regeneration by coke burn-off. Kinetic studies were conducted to verify the influence of mass transfer limitations on the impedance signal.