Passive remote Surface Acoustic Wave (SAW) sensors have been applied e.g. as temperature, pressure or torque sensors. Their important advantages over standard methods are their passive operating principle, which allows operation without any power supply, as well as the wireless high-frequency signal transmission over distances up to about 10…15 m even through (non metallic) housings. These properties of SAW sensors particularly qualify them for applications in high voltage operational equipment. First experience was gained in a long time field test of surge arrester monitoring based on SAW temperature sensors in a German high-voltage substation. Now, this system has been further developed at Darmstadt University of Technology for other applications, the first of them being an overhead line (OHL) conductor temperature measurement, the second one a temperature monitoring system for of high-voltage disconnectors. After designing and building the sensors, extensive laboratory tests were carried out applying high-voltage, high-current and thermal stress in order to approve the suitability for the intended application. All these tests confirmed the assumption that SAW sensors, due to their passive working principle, are not affected at all by any kind of electrical, magnetic or thermal stress that may occur during service. The complete temperature sensor consists of three parts: a sensor chip, an antenna which receives and transmits the signal from and to the radar unit and a body for installation and for protection against environmental impact. One must find a good compromise between optimizing of thermal, dielectric and high-frequency characteristics and at the same time taking into consideration a simple installation. These requirements on the SAW sensors turned out to be difficult to coordinate. To achieve a high measuring precision is especially difficult. First, a new sensor for OHL application was developed. The OHL conductor temperature sensor had been optimized with regard to low-frequency dielectric and high-frequency transmission characteristics. But it has turned out over time that its thermal properties are not optimal. For this reason, the sensor has to be thermally optimized. The problem may be overcome by two alternative or combined approaches: optimization of the thermal parameters of the sensor (thermal capacitance, heat transfer resistances) in order to minimize the influence on the OHL conductor temperature, or corrective calculations based on the raw data of temperature information. Both approaches are actually being investigated. The first approach to a sensor reconfiguration is to design a smaller sensor body, i.e. to minimize its volume, mass and surface area. For this purpose, a new sensor has been developed. The second possibility to improve the thermal properties of the SAW sensors is the correction calculation. By the use of deconvolution and convolution the OHL conductor temperature could be calculated from a known sensor temperature. The correction calculation of the not thermal optimized SAW sensor was tested in the laboratory. It was considered to be possible in principle. In a next step, a couple of sensors have been installed in different field tests in order to test and optimize the entire measuring system under real operation conditions. Both mobile (for occasional temperature measurements) and stationary (for online monitoring purposes) systems were applied. Service experience with this novel OHL temperature measuring system is quite promising so far. Obviously, the technology in hand is well suitable for this kind of application. A second part of the research project covers temperature measurement of high-voltage disconnectors in order to allow condition monitoring of their contact systems. The working principle is the same as for the overhead line temperature measurement, but the sensor itself is a new development as it has to fulfil completely different requirements. The temperature sensor had been optimized with regard to low-frequency dielectric, high-frequency transmission and thermal characteristics. Different types of disconnectors were studied in the laboratory: a 420-kV pantograph disconnector and 245-kV centre break disconnector. At first the positions of the disconnector sensors at these two disconnectors were determined. Different aspects were considered to choose the position. For one, the sensors should not be damaged by electric arc. For another, a secure read of the sensors and a detection of defects should be possible. Afterwards, the thermal properties of the SAW sensor at the chosen position and the possibility to detect defects were studied. In the end two field tests were started.