A flow cell was constructed from plexiglass. The cell was designed to allow insertion and removal of a Clark-type oxygen membrane electrode. It was used in a flow system to amperometrically determine glucose and glucose oxidase via oxygen depletion. Hydrogen peroxide was determined by oxidation at +0.9 V vs. the silver/silver chloride electrode by removing the electrode membrane. Alternatively, a lead dioxide catalyst electrode was constructed and inserted into the cell to catalytically decompose hydrogen peroxide to oxygen, which was detected. Molybdenum (VI) was determined by its catalytic effect on the reaction between hydrogen peroxide and thiosulfate.