Oxygen-ion conducting solid electrolyte cells were prepared with Pt reference electrodes and Fe2O3-Sb2O4 working electrodes. The working electrode served as both electrode and catalyst during the partial oxidation of propene. The electrochemical cell allowed thermodynamic catalyst surface oxygen activities to be followed and acted as a sensor of catalyst state. It is clear that the catalyst was much more selective fur acrolein formation at lower oxygen activities as indicated by the sensor signal. There is evidence to suggest that the oxygen activity measured by the sensor is as a result of the presence of adsorbed oxygen and not lattice oxygen. Based on the results it appears that potentiometric sensors could provide important additional information for catalytic partial oxidation process monitoring and control.