Cresyl violet, a cationic dye (CV+), forms H-aggregates on the negatively charged SiO2 and SnO2 colloids. These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, Ru(bpy)(3)(2+), we are able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal SiO2 and SnO2 suspensions. On SiO2 surfaces, the excited state quenching of RU(bpy)(3)(2+) by the dye-aggregates proceeds via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surface-promoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO2 colloids undergo photosensitized reduction since the excited sensitizer, RU(bpy)(3)(2+)*, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described.