An effective method for determination of the quantum yields of the S-3 --> S-2, S-3 --> S-1 and S-3 --> S-0 intramolecular radiationless transitions, based on steady-state measurements of absorption, S-2 fluorescence and T-1 phosphorescence or S-1 fluorescence under selective excitation to the S-2 and S-3 states has been described and used for three aromatic thioketones. The results show that in the compounds studied the deactivation of the S-3 state is not strictly sequential and the S-3 --> S-1 as well as, unexpectedly, S-3 --> S-0 transitions occur with relatively high yields. Measurement of the quantum yields of S-3 --> S-2, S-3 --> S-1, and S-3 --> S-0 intramolecular radiationless transitions is essential for the determination of the intramolecular properties of molecules in the S-3 state as well as pathways of their decay.