An overall online-monitoring of high-voltage metal-oxide surge arresters (MO arresters) should comprise a surge counter function, an energy monitor and the monitoring of electrical aging if required. It should also be capable of being integrated in existing systems in the substation. These functions, if at all, are realized today by surge counters, control spark gaps, simple leakage current mA-meters or more sophisticated leakage current analysis instruments, e.g. for evaluating the 3^rd harmonic content or the resistive component of the leakage current. All these methods, however, do not really satisfy the demands mentioned above. Each of them has typical technical and/or economical disadvantages. The requirements could easily be fulfilled by a permanent measurement of the arrester temperature on high potential, which has not yet been realized due to the related technical problems, though. A newly developed high-frequency temperature measuring system based on wireless passive surface acoustic wave (SAW) temperature sensors is now offering this possibility for the first time. It is actually being field-tested in an arrester of 360 kV rated voltage and has shown very good performance so far. (4 pages)