UV light-activated highly efficient photocatalytic decomposition of aqueous glucose and polysaccharides (starch and cellulose) to CO2 was successfully achieved by using a mesoporous TiO2 thin film coated on a fluorine-doped transparent conductive glass (FTO). The external quantum efficiency (eta) of 0.08 (=8%) was obtained for glucose photodecomposition at neutral pH based on the total incident UV light, and the internal quantum efficiency (eta') was 8 (=800%) based on the photon that was effective for activating the reactant, demonstrating that the major decomposition mechanism is dark auto-oxidation of the activated reactant by O-2. Glucose gave eta' values of 19 at pH 12 and 25 at pH 2 demonstrating that when a glucose molecule was once activated by one photon, the molecule can undergo auto-oxidative decomposition to CO2 at these pH under dark. Water-soluble starch was also photodecomposed completely to CO2 with estimated eta' value of 8.6. Water-soluble carboxymethyl cellulose (CMC) also underwent decomposition to CO2 with similar efficiency of eta' = 5. Solid state cellulose powders could be photodecomposed to CO2 by sandwiching them between FTO-coated TiO2 thin films.