A simple zirconia sensor equipped with Ag powder electrodes was fabricated, and the response to oxygen in gaseous and aqueous environments was examined at temperatures oner the range from 200 to 330 degrees C. The sensor showed a fast, Nernstian response to changes in oxygen partial pressure in gaseous systems. In aqueous systems at 300 degrees C, the sensor also responded to changes in the dissolved oxygen concentration in a Nerstian manner.