Olive mill wastewater (OMW), a highly polluted wastewater from the olive oil industry, was treated by solar photocatalysis and solar photo-Fenton. Among the tested systems the application of titanium dioxide alone was not successful. The addition of peroxydisulphate as an electron acceptor had only limited effect on degradation performance and led to high salt concentrations (30 g/l sulphate generated) and a pH value near zero. The photo-Fenton method successfully removed up to 85% COD and up to 100% of phenol index of OMW with different initial concentrations and from different sources. Two solar photocatalytic pilot-plant reactors were used; one of conventional CPC type and an open non-concentrating Falling Film Reactor. The latter, newly designed reactor worked properly and yielded comparable results to the CPC in terms of degradation rate referred to incident UV radiation energy per solution volume. The suspended solids in the OMW hinder light from entering the reactor. Therefore, flocculation induced by a commercial flocculation agent was successfully applied to remove suspended solids. Application of this pre-treatment led to considerable increase of degradation rates and decrease of hydrogen peroxide consumption. (C) 2004 Elsevier Ltd. All rights reserved.