We used transient absorption spectroscopy to characterize excited-state electron injection from a 2,7-bis(dimethylamino)-9-(5-phosphonothien-2-yl)selenoxanthylium dye (3-Se) into TiO 2 . Dye 3-Se adsorbed to TiO 2 via the phosphonic acid group as a mixture of H-aggregates and monomers. Injection of electrons from photoexcited 3-Se into TiO 2 yielded the dication radical (3-Se + ) and an associated transient absorption at wavelengths shorter than 540nm, the amplitude of which was proportional to the quantum yield of electron injection (ϕ inj ). Our data revealed that ϕ inj from H-aggregated 3-Se was (2.0±1.3)-fold greater than from monomeric 3-Se; therefore, H-aggregation increased the efficiencies of both light-harvesting and electron injection. Comparison with our reported data for the analogous carboxylic acid-functionalized dye (1-Se) revealed that ϕ inj via the carboxylate linkage was (2.3±1.1)-fold greater than via the phosphonate linkage. Thus, electron-injection reactivity is sensitive to both the aggregation state and the surface-anchoring mode of these chalcogenorhodamine dyes. The decrease of ϕ inj for 3-Se is offset by its enhanced stability and persistence on TiO 2 , rendering the phosphonic acid-functionalized and H-aggregated dye a particularly attractive sensitizer.