Multi-walled carbon nanotubes (MWNT) and nanocrystalline titania composite catalysts were prepared by a modified sol–gel method. Thermogravimetric analysis, N2 adsorption–desorption isotherm measurements, powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and UV–vis spectra were carried out to characterize the composite catalysts with different MWNT contents. The results suggest that the presence of MWNT embedding in the composite catalysts matrix prevents TiO2 particle agglomeration. Additionally, a correlation exists between the MWNT content and the changes in the UV–vis absorption properties. The photocatalytic degradation of phenol was chosen as a model reaction to evaluate the photocatalytic activities of the composite catalysts. An optimum of the synergetic effect was found for a weight ratio MWNT/TiO2 equal to 20%. The effects induced by MWNT on the composite catalysts may be explained in terms of a strong interphase interaction between MWNT and TiO2 in the composite catalysts.