An oxygen sensor based on yttria-stabilized zirconia with Ni-NiO mixture as a solid reference electrode for high temperature applications has been fabricated and tested. During high temperature use Ni reacts with inner Pt electrode and Pt lead wire, and affects the output signal of the sensor leading to a limited lifetime. This is the first time an Al 2 O 3 isolation layer has been introduced in the design of an oxygen sensor. Use of this isolation layer prevents high temperature alloy formation between Ni and Pt, leading to a significant enhancement in the performance and the stability of the sensor. The sensor shows fast response with good recovery characteristics in the temperature range of 600-1400°C. However, after repeated tests over a longer period, the sensor performance degrades possibly due to grain growth and sintering of the inner electrode at high temperature. The measurement and control of oxygen atmosphere inside a ceramic kiln are critical for the manufacture of useful ceramic products.1 Yttria-stabilized zirconia ͑YSZ͒-based oxygen sensors are being used for this purpose. Such sensors can only measure oxygen concentrations in an area very near the wall of the kiln, as the sensor uses an air reference electrode and the length of the zirconia tube limits the location of measurement. Also, in the application environment, the sensor encounters severe operating conditions, such as high temperature, large temperature fluctuations, mixtures of gases, heavy dusts and vapors, and thermal shock. Recognizing the influence of these factors, the sensor probes must be specially designed, particularly those using a solid reference electrode.The use of a galvanic cell for the determination of oxygen concentration in liquid steel was developed in many countries during the decade 1960-1970. 2 At that time a number of metal-metal oxide mixtures ͑e.g., Mo-MoO 2 ,Cr-Cr 2 O 3 , etc.͒ were employed to fix the reference oxygen in a YSZ cell. These cells were exclusively designed for one-time use and oxygen was measured by dipping them in the molten steel. As they were used for one time only, their long-term stability was not an issue. Note here that Cr 3 and Mo 4 react with Pt electrodes and form compounds at relatively lower temperatures. Compound formation eventually leads to an unstable sensor. Solid reference gives an equilibrium oxygen partial pressure at the operating temperature. The main advantage of solid reference over the air is that the metal-metal oxide mixture inherently produces extremely low O 2 concentrations ͑trace͒ with respect to the test environment, thus producing a large pressure difference, leading to enhanced emf signals. If the partial pressure is too low, it is difficult to establish the equilibrium. If too high, the decomposition of metal oxide becomes significant. Thus, choosing a proper metal oxide reference electrode is very critical. Virtually this metal oxide acts as an oxygen pump, which provides a source of steady flow of oxygen. The partial pressure (p O 2 ) generated by a given me...