A novel composite with a core-shell structure (CdS/TiO2) was prepared through the combination of microemulsion and ultrasound (20 kHz). The degradation of reactive black 5 (RB5) was carried out in aqueous solution in a series of experiments by CdS/TiO2 nanoparticles. This composite with mole ratio of 1/6 has shown an exceptional sonocatalytic activity in comparison to the pure nanoparticles of TiO2 and CdS. A significant decrease in the concentration of RB5 (approximate to 94%) was observed in 3 min sonication of the solution containing the core-shell nanocomposite. While at the same time, the concentration was reduced to 4% under sonication without nanocomposite and 50% under UV light with nanocomposite. The increased catalytic activity of nanocomposite in the presence of ultrasound is due to the enhancement of mass transfer, cleaning and sweeping the surface of catalyst, and preventing the aggregation of particles. In addition, the presence of CdS nanoparticles in the composite acts as photosensitizer which not only extends the spectral response to the visible region but also reduces the charge recombination. The selected combined method (sonocatalysis) was able to decolorize and oxidize simultaneously the organic dye with a complete mineralization into SO42- and NO3- ions. (C) 2011 Elsevier B.V. All rights reserved.