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 Al2O3 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 degreesC. 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. (C) 2001 The Electrochemical Society. All rights reserved.