The photophysical changes taking place by coupling of Cd(OH)(2)-coated Q-CdS with colloidal TiO2 have been examined. The coupling of TiO2 causes the quenching of the band gap emission of CdS and the red emission is not affected appreciably. For a typical 0.22 x 10(-3) mol dm(-3) of TiO2 the average emission lifetime of Cd(OH)(2)-coated CdS is reduced from 26.4 to 6.8 ns by trapping of the conduction band electron by TiO2. Cd(OH)(2)-coated Q-CdS does not sensitize the reaction of indole, however, the coupled CdS-TiO2 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 TiO2 which removes Cd(OH)(2) layer to form possibly [CdS-TiO2(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 (phi = 0.18) to yield indigo with a quantum efficiency of 0.08. The concentration of both TiO2 and redox couple controls the extent of charge separation in the photocatalyst. The application of coupled semiconductors as catalysts for photosynthetic work is suggested.