We report a transient absorption study of the kinetics of re-reduction of ruthenium bipyridyl dye cations adsorbed to nanocrystalline TiO₂ films by iodide ions, employing a series of Ru^II(dcbpy)₂X dyes, where dcpby = 4,4′-dicarboxy-2,2′-bipyridyl and X = (NCS)₂, (CN)₂, DTC (diethyldithiocarbamate), and Cl₂. The data indicate that this re-reduction (or ‘regeneration’) reaction proceeds via a transient (dye+-iodide) intermediate complex formed by reaction of photogenerated dye cations with one iodide ion. The subsequent reaction of this complex with a second iodide ion, forming I₂^–, is shown to be the kinetically and thermodynamically limiting step in the overall regeneration reaction. The implications of this reaction scheme for the function of dye sensitized photoelectrochemical cells are discussed, considering in particular the high photovoltaic efficiencies obtained for such devices employing such ruthenium bipyridyl senstiser dyes in combination with iodide/tri-iodide based redox electrolytes.