Composite particles comprised of titanium dioxide strongly coupled to various types of carbon nanotubes (CNTs) were prepared by low temperature hydrothermal synthesis. A significant enhancing effect on the photocatalytic reduction rate of Cr(VI) was observed, as a consequence of Cr(VI) adsorption on the CNTs. This rate-enhancing effect was shown to depend on the type of. the CNTs and on their functionalization. While evidence for the presence of a synergistic effect is well documented, the comparison between the effects of the various types of CNTs enables to present a new phenomenon., connected to the fact that for this specific model system both reduction and oxidation occur mainly on the CNTs. Here, the extent by which the reduction rate will be enhanced depends not only upon the tendency of the CNTs to adsorb the Cr(VI), but also on the ability of the system to shuttle both holes and electrons to the CNTs, namely whether the CNTs are metallic or not. (C) 2013 Elsevier B.V. All rights reserved.