The electrochemical promotion of Pt catalyst electrode films interfaced with Na3Zr2Si2PO12 (NASICON), a sodium ion conductor, was investigated during ethylene oxidation. At 430 degrees C, electrochemical Na+ supply to the Pt catalyst under near-stoichiometric ethylene-to-oxygen ratios causes an up to tenfold catalytic rate enhancement for Na coverages of 0.03-0.08. The behavior of the system is discussed in terms of previous electrochemical promotion studies using sodium-ion conductors. On the basis of the kinetic data the promotional role of sodium is attributed to enhanced oxygen chemisorption on the Pt catalyst-electrode surface.