The coordination of halide ions to 5-(3,5-dicarboxyphenyl)-10,15,20-tri-p-tolylporphinatozinc(II) anchored to mesoporous nanocrystalline (anatase) TiO2 thin films (TiO2/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 TiO2/ZnP and TiO2/ZnP-X-: A(Soret)ZnP = 427 nm (epsilon = 574 000 M-1 cm(-1)), A(Soret)ZnP-Cl- = 435 nm (epsilon = 905 000 +/- 12 000 M-1 cm(-1)), A(Soret)ZnP-Br- = 436 nm (epsilon = 776 000 +/- 30 000 M-1 cm(-1)), and A(Soret)ZnP-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 TiO2/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 TiO2/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-Cl0/-) = +630 mV. In regenerative solar cells with quinone/hydroquinone redox mediators, TiO2/ZnP and TiO2/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.