Porous nanostructured photoanodes prepared from colloidal solutions of DeGussa or sol-gel processed TiO2 material have been derivatized with cis-M(dcb)(2)(CN)(2), where M = Ru(II) or Os(II) and dcb is 4,4＇-(COOH)(2)-2,2＇-bipyridine. When employed in a regenerative solar cell with iodide donors, cis-Ru(dcb)(2)(CN)(2)/TiO2 converts absorbed photons to electrons nearly quantitatively while cis-Os(dcb)(2)(CN)(2)/TiO2 does not. In-situ time-resolved diffuse reflectance measurements reveal a sluggish iodide oxidation rate is responsible for the low photocurrent efficiency of cis-Os(dcb)(2)(CN)(2)/TiO2. Transient absorption measurements support this conclusion and allow full wavelength characterization of the charge-transfer processes.