Thin films of SnO2 nanocrystallites have been surface-modified with thionine, methylene blue, and oxazine 170 by adsorption from the corresponding dye solutions. The strong electrostatic interaction between the cationic dye and the negatively charged semiconductor nanocrystallites results in close packing of the dye on the semiconductor surface. These closely packed H-aggregates of the adsorbed dye are active both electrochemically and photoelectrochemically. Electron transfer from semiconductor nanocrystallites into the adsorbed dye aggregates leads to bleaching of the colored film. The extent of dye bleaching which is readily controlled by the applied potential, has been probed by spectro electrochemical measurements. The photocurrent action spectra of these dye-modified SnO2 films indicate charge injection from excited dye aggregate into the semiconductor nanocrystallites with an incident photon-to-photocurrent efficiency of <1%.