Laser photolysis of TiO2 porous layers has been studied using iodide as a hole-scavenger. The I-2(-) transient product, which is produced inside the layer, decays predominantly by the second-order reaction with TiO2 electrons and by disproportionation, The resulting I-3(-) ion also reacts with the TiO2 electron, although the rate of reaction is 2 orders of magnitude slower than I-2(-) Evaluation of the rate constants was achieved by fitting computed curves of absorbance vs time profiles to the experimental data. The rate constants k(e(TiO2)(-) + I-2(-)) (2.25 +/- 0.6) x 10(5), k(e(TiO2)(-) + I-3(-)) = (1.65 +/- 0.4) x 10(3), and k(I-2(-) + I-2(-)) = (1.1 +/- 0.15) x 10(5) M-1 s(-1) have been determined in the TiO2 layer. The rate constants suggest relatively low diffusion rates for I-2(-) radical ion and e(-)TiO(2). These have been attributed to electron trapping at nanocrystallite boundaries as well as activation and steric barriers. The rate of reaction of I-3(-) With the TiO2 electron suggests that this reaction must be important in photoelectrochemical cells involving iodide and iodine. The high photocurrent yields observed in some of these cells, is attributed to dense coverage of the TiO2 surface by the photosensitizer.