A relaxation method is described which allows a direct measurement of radiationless transition rates. The method corresponds to the spectrophone in the gas phase; i.e. it employs intensity modulated light of variable modulation frequency as the means for perturbing the equilibrium, and a microphone as the measuring device for detecting the resultant pressure wave. By applying the method to solutions of eosine in methanol the following results were obtained: 1) The rate of the radiationless transition from the first excited singlet state of eosine to the triplet state is 1.25 · 108 sec−1. 2) Besides this transition the radiationless deactivation of the first excited singlet state to the ground state may be neglected. 3) The natural lifetime of the triplet state of eosine is 0.104 sec. 4) Even at low temperatures (‐196°C) the radiationless deactivation of the triplet state is much more prominent than the radiative transition. Generalizations of the optic‐acoustic relaxation method are discussed.