To determine the oxygen storage capacity of three-way catalysts in gasoline-operated vehicles, lambda probes are best practice to measure the A/F ratio of the exhaust for monitoring indirectly the conversion efficiency. In this paper, initial tests of a novel approach are described to determine the oxygen loading of the TWC more directly by means of radio frequency without the use of lambda probes. Experiments in synthetic exhaust gas as well as dynamometer tests using ceria-containing commercial catalysts showed that characteristic parameters of the radio-frequency signal coincide with the oxidation/reduction state of the TWC.