A new, simple and highly sensitive spectroscopic method, calorimetric absorption spectroscopy (CAS), for direct detection of phonons emitted via nonradiative recombination processes of charge carriers bound to defects or impurities in solids is demonstrated. Electronic levels of these centers are populated by resonant excitation with monochromatic light. The phonons emitted during recombination are detected with a low‐temperature calorimeter with a sensitivity of ≊10−12J. Thus absorption coefficients as low as ≊10−7cm−1and quantum efficiencies can be determined without using radiation detectors. CAS spectra of N‐  and S‐doped GaP are presented, and the low‐temperature internal quantum efficiency of the nitrogen bound exciton is found to be close to 100%.