There is a significant need for sensors capable of detecting gases, such as H<
sub>
2<
/sub>
, O<
sub>
2<
/sub>
, NO<
sub>
x<
/sub>
, SO<
sub>
x<
/sub>
within harsh environments encountered in power plants, industrial manufacturing, oil and gas exploration, and aerospace applications. This project successfully demonstrated the use of wireless microwave acoustic sensor technology for the detection of gases (H<
sub>
2<
/sub>
or O<
sub>
2<
/sub>
) from ambient temperatures up to 650�C. The work focused on langasite (LGS) based surface acoustic wave resonator (SAWR) sensors as the harsh-environment sensor platform and explored multiple combinations of high-temperature thin films and device structures which were used to increase the sensor platform stability and detection capability at temperatures in the operational range of 150�C to 700�C. Specific material configurations that were investigated include: yttria-stabilized-zirconia (YSZ) decorated with Pt nanoparticles, atomic layer deposited (ALD) Al<
sub>
2<
/sub>
O<
sub>
3<
/sub>
, palladium, and Pt/Al<
sub>
2<
/sub>
O<
sub>
3<
/sub>
co-deposited electrode alloys. Through the deposition of YSZ at temperatures as high as 850�C and the use of graded alloy concentrations of Pt in the fabrication of the Pt/Al<
sub>
2<
/sub>
O<
sub>
3<
/sub>
electrode structures, film stress problems were mitigated, and sensor operation and stability achieved. To test and evaluate SAWR sensor performance for the detection of H<
sub>
2<
/sub>
and O<
sub>
2<
/sub>
under the influence of temperature variations, a comprehensive gas sensor control system and test apparatus was created to operate within a laboratory box furnace-controlled environment. In addition to the advancement in thin film materials through the deposition and fabrication techniques mentioned above, the work characterized the performance of sensors containing these films in the presence of oxidizing and reducing gases between 25�C and 700�C. In particular, the work revealed that the exposure of the SAWR sensor surfaces to oxidizing environments significantly improve the sensor response to H<
sub>
2<
/sub>
, whereas the exposure of the sensor to reducing environments at high temperatures (? 500�C) renders the sensor irresponsive to H<
sub>
2<
/sub>
, requiring sensor surface treatment at high temperatures (above 500�C) to recover the responsiveness to H<
sub>
2<
/sub>
. The SAWR sensors have been also tested for wireless operation and array operation using multiple orientations to resolve the detection of gases under temperature variations. The work developed at the University of Maine was aided by a collaboration with the NETL Research and Innovation Center, Pittsburgh, PA, where thin film materials and device structures fabricated at UMaine were tested and characterized using NETL gas reactors and surface analysis techniques. SAWR sensors fabricated at UMaine were exposed multiple times to temperatures up to 700�C and H<
sub>
2<
/sub>
concentrations up to 100% in the NETL facilities to measure the sensor performance. Environetix Technologies Corporation, a UMaine harsh-environment sensor spin-off company, also provided support and assistance in sensor system testing and implementation. The sensor small size and configuration allows flexible sensor placement and embedding of multiple sensor arrays into a variety of components within power systems and other aerospace or industrial settings that need to be interrogated wirelessly. The SAW platform is an attractive option for high-temperature harsh-environment gas sensing applications due to its inherent features, namely small size, capability of battery-free and wireless operation, and cost effective scale production using well-established production techniques from the semiconductor industry. The research findings achieved in this work, particularly advances regarding the fabrication and performance of the SAWR gas sensor platform, can be adapted and transferred to industrial power plants and other harsh environments.