Oxygen-ion conducting solid electrolyte electrochemical cells were prepared with both Fe2O3-Sb2O4 and Pt working electrodes. The working electrode served as both catalyst, for a heterogeneous catalytic reaction taking place in the electrode compartment, and electrode. With the Fe2O3-Sb2O4 electrode, the electrochemical cell was used as a potentiometric sensor to determine surface oxygen activities during the partial oxidation of propene to acrolein. The sensor was found to be selective to surface oxygen involved in the competing total oxidation reaction. With the Pt working electrode, the cell was operated in a closed-circuit mode with oxygen being electrochemically supplied and removed from the catalyst surface over which the oxidation of carbon monoxide was being performed. The large changes in reaction rate compared to the rate of electrochemical oxygen supply were explained by the electrochemical reaction having different selectivities, for the formation of surface oxygen species, from the purely surface reaction.