The coordination of halide ions to 5-(3,5-dicarboxyphenyl)-10,15,20-tri- p-tolylporphinatozinc(II) anchored to mesoporous nanocrystalline (anatase) TiO 2 thin films (TiO 2/ZnP) immersed in propylene carbonate was quantified. The addition of tetrabutylammonium halide salts to the external propylene carbonate electrolyte resulted in a red shift in the absorption spectrum with the maintenance of five isosbestic points. The absorption spectra were within experimental error the same for ZnP and ZnP-X (-) compared to TiO 2/ZnP and TiO 2/ZnP-X (-): A SoretZnP = 427 nm (epsilon = 574 000 M (-1) cm (-1)), A SoretZnP-Cl (-) = 435 nm (epsilon = 905 000 +/- 12 000 M (-1) cm (-1)), A SoretZnP-Br (-) = 436 nm (epsilon = 776 000 +/- 30 000 M (-1) cm (-1)), and A SoretZnP-I (-) = 437 nm (epsilon = 620 000 +/- 56 000 M (-1) cm (-1)). Titration studies with the halides revealed sharp isosbestic points consistent with formation of a 1:1 halide/porphyrin adduct. Equilibrium constants for ZnP were found to be 1670 M (-1) for Cl (-), 96 M (-1) for Br (-), and 5.5 M (-1) for I (-), and the corresponding values for TiO 2/ZnP were significantly smaller, 780 M (-1), 70 M (-1) and 3.4 M (-1). A quasi-reversible wave was observed by cyclic voltammetry of TiO 2/ZnP, E 1/2(ZnP (+/0)) = +790 mV vs Ag/AgCl, that was shifted 160 mV after addition of excess chloride, E 1/2(ZnP-Cl (0/-)) = +630 mV. In regenerative solar cells with quinone/hydroquinone redox mediators, TiO 2/ZnP and TiO 2/ZnP-X (-), where X is Cl, Br, or I, were found to convert light into electrical power. The photocurrent action spectrum demonstrated that energy conversion was initiated by light absorption of ZnP and/or the halide adduct.