The interactions between Pd/TiO2 catalyst and the reactants and potential reaction intermediates present during aqueous nitrate reduction, including NO3-, NO2- and NO in the presence of H-2 and H2O were studied by infrared spectroscopy. Adsorbed forms of NO, nitrite and nitrate could all be detected in the presence of water. In the presence of water/H-2, nitrate was the most stable surface species followed by nitrite and then highly reactive NO, suggesting that the reduction of nitrate to nitrite is the rate-limiting step. High concentrations of adsorbed nitrite appear to be linked to the detection of gaseous N2O while the formation of ammonia is related to reactions on the Pd surface and the extent of formation is linked to high levels of adsorbed NO in addition to the surface hydrogen availability and the presence of water. (C) 2007 Elsevier B.V. All rights reserved.