Particulate TiO2 layers of various thickness were prepared by sedimentation from suspension and investigated by SEM, UV-vis spectroscopy, photocurrents and photodegradation rate measurement. In pure electrolyte (Na2SO4), the photocurrent reaches a maximum around 0.5 mu m and then rapidly decreases with increasing thickness. The explanation consists in the fact that with increasing layer thickness the average distance of photogenerated electrons and holes from back contact of electrode increases. This results in higher recombination and lower electron flux (photocurrent) to the back contact. In the presence of oxalic acid, photocurrent is about one-order higher than in Na2SO4 and it does not decrease with layer thickness. It can be attributed to the strong adsorption of oxalic acid, which is directly oxidized by holes. The stationary concentration of holes is thus very small and the probability that electron/hole pairs will recombine is much lower than in pure electrolyte and does not depend too much on the distance from the conducting substrate. (C) 2005 Elsevier Ltd. All rights reserved.