A thin film polarographic oxygen sensor, in which a number of thin film microcathodes are deposited in holes in a thin film layer of silicon dioxide formed on a silicon substrate. A single thin film anode layer is deposited on the insulating layer and is insulated thereby from the cathodes. Preferably the microcathodes and the anode are located in a well in the thin film insulating layer, the well being filled flush to its surface with electrolyte, and having a thin film polymer filter membrane deposited thereover. Contacts to the anode and cathodes may be punched through a thin film insulating layer on the back of the wafer, the cathode contact being thereby connected to the substrate, and the anode contact being insulated from the substrate by an encircling thin film layer of silicon dioxide and extending through to the anode metallization layer. With appropriate choice of electrode, electrolyte and membrane materials, the cell may also be used as a pH sensor, a CO.sub.2 sensor, a specific ion sensor, or to detect or sense other substances in solution. Single electrode units may also be formed by depositing thin film electrode materials on appropriate substrates.