It is shown for the first time that the recently developed calorimetric absorption spectroscopy (CAS) is most fruitfully applicable as a direct measure of nonradiative decay of excitonic systems even concerning their fine structure properties if experiments are performed in the mK range achieved in a3He/4He‐dilution refrigerator. CAS gives hints on different scattering and relaxation processes of longitudinalALand tripletAFexcitons in CdS, and directly evidences nonradiative decay ratios of different bound‐exciton complexes. Even TA‐phonon‐assisted recombination of such systems leads to well measurable CAS signal enhancement.