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A novel absorption technique, calorimetric absorption spectroscopy (CAS), is described and modeled in detail. The technique is more sensitive than any other low‐temperature absorption technique reported hitherto and it is quantitative. Quantum efficiencies can be determined by combining CAS with calorimetric transmission spectroscopy. The method is based on integral detection of phonons emitted during nonradiative recombination processes. A low‐temperature carbon resistor acts as phonon detector. The sensitivity of CAS is so large that a simple combination of a tungsten lamp and a spectrometer can be used as the illumination source. Depending on the excitation density, αd products down to 10-8 can be detected. These features make it possible to apply CAS for the characterization of the interface structure of single quantum wells with thicknesses of less than 1 nm as well as for the study of the fine structure of deep traps in semiconductors. Examples for all these applications are given.