- Small-sized radiation scintillation detectors have been extensively developed for portable radiation survey systems, for a variety of applications. The CsI(Tl) capacity to discriminate the energy-lines of a gamma-radiation source makes these detectors suitable for personal detection and spectrometry. Additional advantages are their miniature electronics and operation at room temperature. However, it is essential to inspect their efficiency, sensitivity and stability under the practical range of the environmental conditions expected during operation. The aim of this research is to study the changes in the response of a CsI(Tl) detector, 22 mm in diameter and 25 mm long, in the full range of operation temperatures, from -20 to 60 degC . Results show that the photopeak efficiency for the Cs-137 gamma-line remains constant, even though the peak center channel is shifted as function of the detector temperature. The peak spectral resolution was also analyzed in the same temperature range and was found to be around 8%. Since the photopeak efficiency has been constant over the entire tested temperature range, the CsI(Tl) detector is a suitable spectrometry detector for environmental operations.