EPR measurements demonstrate efficient charge separation on carotenoid-modified titanium dioxide nanoparticles (7 nn). Strong complexation of carotenoids containing terminal carboxy groups (-CO2H) with the TiO2 surface leads to electron transfer from the adsorbed carotenoid molecule to the surface trapping site. For these systems, EPR signals of the carotenoid radical cations Car(.+) and the electrons trapped on the TiO2 are observed before irradiation (77 K). Their UV-visible spectra show an absorption band with a maximum near 650 nm that is characteristic of the trapped electrons. Surface modification of the TiO2 by other carotenoids results in the formation of a complex with an optical absorption band near 545 nm. These systems form charge-separated pairs [Car(.+)... TiO2(e(tr)(-))(surf). TiO2(e(tr)(-))(latt.)] only upon 365-600 nm illumination at 77 K. Complexation of the TiO2 colloids with carotenoids enhances spatial charge separation, shifts the absorption threshold; into the visible region, and thus greatly improves the reducing ability of the semiconductor. Photoreduction of acceptor molecules such as 2,5-dichloro-1,4-benzoquinone, nitrobenzene, and oxygen is demonstrated.