Abstract: A apparatus and method for the reduction and oxidation of constituents of a fluid is disclosed. The apparatus comprises a honeycomb-like structure of oxygen ion conducting ceramic having at least one passage. The passage is lined with a porous bipolar electrode and porous electrodes are formed outside the passage on the ceramic structure, the electrode in the passage being between the outside electrodes. If a voltage is applied to the outside electrodes, reducible and oxidizable constituents in a fluid passing over the passage electrode are oxidized and reduced. The capacity of the apparatus may be increased by increasing the number of passages/electrodes located between the outside electrodes.