Silicon-on-insulator MESFETs have been manufactured using a commercial SOI CMOS process and their electrical characteristics measured from room temperature up to 200°C. No modifications were made to the CMOS process flow. The prototype devices use a CoSi2 gate material and the gate current follows the expected Shottky diode behavior. At room temperature a 0.6 μm gate length device has a threshold voltage of - 0.8 V with an off-state drain current of approximately 5 nA. The device shows an attractive family of I-V curves up to 200°C. For higher temperatures the reverse diode current makes it hard to switch the device off. Numerical simulations of a similar device with a higher barrier height PtSi gate show reasonable behavior up to 300°C.