Nanostructured composite catalysts produced using multi-walled carbon nanotubes (CNT) and titanium dioxide were prepared by means of a modified acid catalyzed sol-gel method. Materials were extensively characterized by XRD,TG, N-2 adsorption-desorption isotherms, TEM, UV-vis spectroscopy and TPD. Composite catalysts with different CNT contents were tested in the photo-oxidation of phenol under different irradiation wavelengths. A correlation between CNT content and the changes in the UV-vis absorption properties is observed especially in the visible spectral range. The introduction of CNT into the titanium matrix induced a synergic effect when irradiating in the near-UV to visible spectral range. This effect may be explained in terms of a strong interphase interaction between CNT and TiO2 in the composite catalysts. CNT with different surface chemical groups were used to produce CNT-TiO2 composites. The presence of mild concentrations of oxygen containing surface groups, mainly carboxylic acids and phenols, appears to produce a beneficial effect in the photocatalytic activity of CNT-TiO2 composite catalysts. The photocatalytic oxidation of other monosubstituted benzene derivatives such as, aniline, nitrobenzene and benzoic acid, has been investigated under visible irradiation. The primary intermediates of photocatalytic reaction are single substituted hydroxy derivatives. The photoefficiency of the composite catalyst showed to be affected by the aromatic ring reactivity properties of the substrate. (C) 2010 Elsevier B.V. All rights reserved.