Humic acids (HA) are known to be present at a high concentration in leachates from "mature" and "old" landfills. In this work, the photocatalytic degradation of Aldrich-Sigma HA as a model of refractory organic of stabilised leachate was studied. The experiments were carried out in HA aqueous suspension of TiO2 (P-25) in CPC reactor at pilot-plant scale in the Plataforma Solar de Almeria (Spain) under solar irradiation and natural pH. The adequate quantity of catalyst for this process was found to be 0.7 g(1-1). The effect of inorganic ions, commonly present in real leachate such as HCO3-, SO42-, Cl- on photocatalytic degradation of HA was also studied. The ions examined, even in higher concentration for Cl- (4.5 g/l) and SO42- (7.75 g/l) were found to not affect the photocatalytic process and the similar efficiency of HA removal was ensured. However, the presence of hydrogen carbonate into solution matrix inhibited strongly the photocatalytic reaction. The catalytic oxidation of HA, in particular in the mixture of anions, did not agree with Langumir-Hinshelwood kinetics and the evaluation of results was expressed only as an effect of HA removal versus accumulated energy (Q(UV), kJ 1(-1)) by photoreactor. Moreover, the effect inorganic matrix on initial adsorption of HA has been discussed on the basis of the results obtained in laboratory batch test. Additionally, the biodegradability evolution during photodegradation of HA was evaluated in terms of biological oxygen demand (BOD5) as well as by oxygen uptake rate. (C) 2004 Elsevier B.V. All rights reserved.