The light response of cesium iodide crystals was measured at temperatures of 303 K (30 °C) to 203 K (−70 °C) and for gamma sources with energies from 17 keV to 835 keV. Two different methods were used to determine the properties of the CsI:Tl crystals. The first used classic analogue spectrometry equipment to check the dependence of energy resolution, intrinsic resolution, and non-proportionality parameters as a variations in temperature, excitation energy, and integration time. The second method registered the light pulses from the PMT without integrating or shaping circuits. At the current stage of the research, the authors have decided to maintain the thesis about the existence of three decay components in CsI:Tl crystals that was employed in previous studies - the light pulse decays were analyzed under this assumption. We observed that the light pulses slowed with a decrease in temperature. When the crystals were cooled from 303 K to 203 K, the fitted decay times increased significantly by factors of two to four. The non-proportionality of the three fitted components was also determined and it was found that the first and second (fast and slow components) exhibited positive non-proportionality while the third (tail component) presented a negative trend for non-proportionality at temperatures of 303 K–203 K. It was found that using sufficiently long integration times, to include compensating trends of all decay components, the light response of CsI:Tl could be proportional.