The photophysical changes taking place by coupling of Cd(OH) 2 -coated Q-CdS with colloidal TiO 2 have been examined. The coupling of TiO 2 causes the quenching of the band gap emission of CdS and the red emission is not affected appreciably. For a typical 0.22x10 - 3 moldm - 3 of TiO 2 the average emission lifetime of Cd(OH) 2 -coated CdS is reduced from 26.4 to 6.8ns by trapping of the conduction band electron by TiO 2 . Cd(OH) 2 -coated Q-CdS does not sensitize the reaction of indole, however, the coupled CdS-TiO 2 catalyst photoinitiated this reaction efficiently. The photoactivity of the coupled semiconductor is suggested to increase due to chemical interaction between Cd(OH) 2 -coated Q-CdS and TiO 2 which removes Cd(OH) 2 layer to form possibly [CdS-TiO 2 (OH) 2 ]. The photogenerated hole in the coupled catalyst CdS(h + ) forms an emissive exciplex with indole. The exciplex is long lived and results in the decomposition of indole (φ=0.18) to yield indigo with a quantum efficiency of 0.08.The concentration of both TiO 2 and redox couple controls the extent of charge separation in the photocatalyst. The application of coupled semiconductors as catalysts for photosynthetic work is suggested.