We present three ultra-low-power CMOS circuits: a temperature sensor, a voltage reference and a comparator developed for an ultra-low-power microsystem (ULP-MST) aiming at temperature sensing in harsh environments. The microsystem has 3 main functions: detecting a user-defined temperature threshold T 0, generating a wake-up signal that turns on a data-acquisition microprocessor (located in a safe area) above T0, and measuring temperatures above T0. To achieve ultra-low-power operation, the three CMOS circuits are implemented in Silicon-on-Insulator (SOI) CMOS technology and are optimized to work in the subthreshold regime of the transistors. Since our application is mainly for harsh environment (i.e. high temperature and radiation), the chip has been designed using a suitable 1-μm SOI-CMOS technology. Simulations have been performed over the different process corners to verify functionality after fabrication. The typical power dissipation at high temperature (up to 240°C) is less than 100 μW at 5 V supply voltage. Measurements have validated correct operation in the temperature range from -40°C to 300°C before radiation and to 125°C after radiation up to now which will be extended further with a new set-up. Irradiation has been performed from 10 to 30 kGy. Such very high doses cause a shift down of output voltage values, which leads to a change of the temperature detection level and also increases the power dissipation by up to six times. Annealing effects help the partial recovery of the device operation at high temperature and the remote microprocessor enables calibration after radiation to readjust the temperature detection level.