Anomalous results obtained while using the Kelvin method of determining contact potential differences between oxide-coated cathodes and an evaporated gold film reference surface in an atmosphere of hydrogen have been investigated. Very high contact potential differences have been associated with a separation of charge from ionized gas on the passage of a discharge current. This charge, either positive ions or electrons, remains on the surface of the oxide cathode for periods (sometimes as long as several days) dependent on the conductivity of the oxide coating. This in turn depends upon the nature of the oxide material, its state of activation and its temperature. Experiments are also described on the behaviour of probe cathodes in hydrogen.