In the present work, a theoretical study on the deactivation of triplet excited (T-1) state thioxanthone (TX) by indole (INH) was performed, based on density functional theory calculations. Three feasible pathways, namely direct electron transfer from INH to T-1 state TX, electron transfer followed by proton transfer from INH center dot+ to TX center dot-, and H-atom transfer from nitrogen of INH to keto oxygen of T-1 state TX, were proposed theoretically to be involved in T-1 state TX deactivation by INH.