SiO2 and SnO2 colloids are capped with a cationic dye, rhodamine 6G, by electrostatic interaction. The close packing of these dye molecules on the negatively charged SiO2 or SnO2 colloid results in the formation of H-aggregates. These aggregates are nonfluorescent but can inject electrons from the excited state into SnO2 colloids. The photophysical and photochemical properties of rhodamine-6G-aggregate on SiO2 and SnO2 colloids have been investigated using picosecond laser flash photolysis. Charge injection from the excited dye aggregate into SnO2 nanocrystallites occurs with a rate constant of 5.5 x 10(9) s(-1). The application of these dye aggregates in extending the photoresponse of nanocrystalline SnO2 film has been demonstrated by constructing a photoelectrochemical cell. A maximum incident photon-to-photocurrent efficiency of similar to 1% was observed for the photosensitized current generation. Fast reverse electron transfer between the injected electron and the cation radical of the dye aggregate is a limiting factor in maximizing the incident photon-to-photocurrent efficiency (IPCE).