Steady state S-2-S-0 and S-1-S-0 absorption and emission spectra and picosecond S-2 decay and S-1 fluorescence rise times have been measured for the model porphyrin ZnTPPS4 in water and in aqueous iodide solutions of constant ionic strength. The dynamics of S-1 quenching by iodide are well-modeled by a Stern-Volmer mechanism yielding k(Q) = 1.75 x 10(9) M-1 s(-1). The S-2 state is quenched on a ps time scale by a static electron-transfer mechanism at an average donor-acceptor separation of 0.38 nm. The implications of these data for dye-sensitized solar cells that employ I/I-3 as the redox medium are discussed. (C) 2010 Elsevier B.V. All rights reserved.