The influence of electrolyte pH on the sensitization efficiency of porphyrin-derivatized TiO2 was studied by photoelectrochemical and transient absorption measurements. The porphyrins 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatozinc(II) (1), 5-(4-carboxyphenyl)-10,15,20-trimesitylporphine (2), 5-(4-carboxyphenyl)10,15,20-trimesitylporphinatoplatinum(II) (3), and 5-(4-dihydroxyphosphorylphenyl)-10,15,20-trimesitylporphinatozinc(II) (4) were anchored to low-surface-area and nanocrystalline TiO2 films. The TiO2 conduction band edge potential (E-CB) shifted 59 +/- 2 mV/pH, from -0.43 V vs Ag/AgCl((aq)) at pH 12 to + 0.16 V vs Ag/AgCl((aq)) at pH 2. Excited-state potentials (E-1/2(P+'*)) Of 1-4 ranged from - 1.58 to -0.91 V vs Ag/AgCl(aq), well negative of E-CB. Despite the thermodynamic favorability of electron injection, a 10-fold increase in sensitized photocurrent was measured for 1-4 upon acidification of the electrolyte from pH 10 to 4. Transient absorption data revealed that sensitization of nanocrystalline TiO2 by 1-4 depended on pH in an identical manner. A mechanism is proposed wherein protonation of a surface site is required for charge compensation of injected electrons. Thus, the magnitude of sensitized photocurrent is determined by surface protonation-deprotonation equilibria.