In order to investigate the origin of the effect of non faradaic electrochemical modification of catalytic activity (NEMCA), a Kelvin probe was used to measure in situ the changes induced in the work function of an IrO2 catalyst film deposited on yttria-stabilized zirconia upon electrochemical supply of O2- to the catalyst under reaction conditions. Ethylene oxidation was chosen as a model reaction system. For this purpose an electrochemical reactor of novel design was used in which work function measurements could be carried out in situ during kinetic measurements. It was found that the changes in catalyst work function equal to changes in catalyst ohmic-drop-free potential and that the reaction rate depends exponentially on catalyst work function at low applied potentials.