In this study the different processes occurring on the TiO2 and Pd and Cu doped TiO2 surfaces during the photocatalytic formic acid degradation in the presence or absence of S2O82- or H2O2 have been investigated. FTIR studies have shown the relevance of the H-bounded hydroxyl groups. It has been observed that formate molecules interact simultaneously with the dopants and surfacial Ti atoms yielding an intermediate species which plays an important role in the phototacatalytic degradation mechanism. Also, it has been determined that Pd or Cu oxides may act as receptors or transmitters of the TiO2 photogenerated electrons and thus modify the degradation mechanism. Different redox reactions have been proposed according to the obtained results. (C) 2004 Elsevier B.V. All rights reserved.