The photophysical and photochemical behaviors of thioxanthen-9-one (TX)in different solvents have been stud-ied using nanosecond transient ab-sorption spectroscopy.A unique ab-sorption of the triplet state 3TX* is observed,which involves two compo-nents,3nπ* and 3ππ* states.The 3ππ* component contributes more to the 3TX* when increasing the solvent polarity.The seli-quenching rate constant ksq of 3TX* is decreased in the order of CH3CN,CH3CN/CH3OH(1∶1),and CH3CN/H2O (1∶1),which might be caused by the exciplex formed from hydrogen bond interaction.In the presence of diphenylamine (DPA),the quenching of 3TX* happens efficiently via electron transfer,producing the TX·-anion and DPA· + cation radicals.Be-cause of insignificant solvent effects on the electron transfer,the electron affinity of the 3nTπ*state is proved to be approximately equal to that of the 3ππ* state.However,a solvent de-pendence is found in the dynamic decay of TX·-anion radical.In the strongly acid aqueous acetonitrile (pH=3.0),a dynamic equilibrium between protonated and unprotonated TX is definitely observed.Once photolysis,3TXH+* is produced,which contributes to the new band at 520 nm.