We have employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of the ruthenium dye RuII(2,2‘-bipyridyl-4,4‘-dicarboxylate)2(NCS)2 (1) adsorbed onto the surface of nanocrystalline titanium dioxide (TiO2) films. This sensitizer dye is of particular interest as it is the most efficient sensitizer dye reported to date and is receiving considerable attention for applications in photoelectrochemical solar energy conversion. Transient data collected for 1 adsorbed onto TiO2 films were compared with those obtained for control dye-coated ZrO2 films, as the high conduction band edge of ZrO2 prevents electron injection. Adsorption of the dye onto the TiO2 film was found to result in a rapid (<500 ps) quenching of the dye excited-state luminescence. Absorption difference spectra collected for the two dye-coated films were assigned by comparison with the spectroscopy of the dye excited and cation states in solution. These transient absorption data...