The photocatalytic activity of CdS nanocrystallites (n-CdS) supported by TiO2 (P-25) nanocrystallites was higher for reductive dechlorination of 1,2,3,4-tetrachlorobenzene than that of n-CdS alone under visible light irradiation. This synergistic effect of TiO2 could be explained as being due to the adsorptive interaction of triethylamine (TEA) at the CdS/TiO2 interface, which contributed to the effective quenching of the holes formed in n-CdS by TEA.