SnO2 colloids are capped with the cationic dyes thionine and cresyl violet by electrostatic interaction. The close packing of these dyes on the negatively charged SnO2 colloids resulted in the formation of H-aggregates. One-electron reduction and oxidation of these H-aggregates bound to SnO2 colloids has been investigated by pulse radiolysis. The spectral features of semireduced, (OH)-O-.-adduct, and semioxidized forms of dye monomers and aggregates are compared by recording time-resolved transient absorption spectra. The bimolecular rate constants for the reaction of dye-capped SnO2 colloids with aqueous electrons (5-6 x 10(9) M(-1) s(-1)) and (OH)-O-. radicals (3.5 x 10(8) M(-1) s(-1)) are nearly an order of magnitude smaller than those for the corresponding reactions of dye monomers. The slower diffusion of colloidal particles is likely to influence the diffusion kinetics of reactions involving dye-capped SnO2 colloids.