Photoirradiation of TiO2 nanoparticles by visible light in the presence of the water-soluble natural polysaccharide arabinogalactan complexes of the hydrocarbon carotenoid beta-carotene leads to enhanced yield of the reactive hydroxyl (OH) radicals The electron paramagnetic resonance (EPR) spin-trapping technique using a phenyl N-tert-butyl nitrone (PBN) as the spin-trap has been applied to detect this intermediate by trapping the methyl and methoxy radicals generated upon reaction of the hydroxyl radical with dimethylsulfoxide (DMSO) The free radicals formed in this system proceed via oxygen reduction and not via the reaction of holes on the TiO2 surface As compared with pure carotenoids carotenoid-arabinogalactan complexes exhibit an enhanced quantum yield of free radicals and stability toward photodegradation The observed enhancement of the photocatalytic efficiency for carotenoid complexes as measured by the quantum yield of the desired spin adducts arises specifically from the decrease in the rate constant for the back electron transfer to the carotenoid radical cation These results are Important for a variety of TiO2 applications namely in photodynamic therapy and in the design of artificial light-harvesting, photoredox, and catalytic devices