In our continuing efforts to surface-modify semiconductor colloids, we have succeeded in preparing TiO2-capped SnO2 (SnO2@TiO2) and TiO2-capped SiO2 (SiO2@TiO2) colloids. The SnO2@TiO2 colloids are 80-100 Angstrom in diameter and exhibit improved photochromic and photocatalytic efficiencies compared to the native colloids. The improved charge separation in this system was confirmed from the enhanced efficiency of hole trapping monitored from the absorption peak at 360 nm. The photocatalytic properties of SiO2@TiO2 colloids are similar to that of native TiO2 colloids. The capped semiconductor systems are useful for the oxidation of I- and SCN-. For example, the quantum efficiency for I- oxidation can be improved by a factor of 2-3 upon capping the SnO2 colloids with TiO2. The distinction between the capped and coupled semiconductor systems has been made by preparing nanocrystalline thin films in two different geometries and studying their photoelectrochemical behavior.