Quantum efficiency of transparent thin film titania photocatalysts under constant photon irradiation during room-temperature water decomposition reaction without external bias potential was measured for the first time as the wavelength was scanned between 250 and 1200 nm. It decreased as the crystal structure of titania was systematically varied from anatase to mixtures of anatase and rutile and then to rutile. The same trend was observed for photocurrents measured under external bias potential. These results indicate structure sensitivity of the reaction and intrinsic advantage of anatase over rutile. The anatase sample tested in a single unit photoelectrocatalysis (SUPEC) cell produces hydrogen at the rates of 0.42 1 h(-1) m(-2) without bias under sunlight and 26 1 h(-1) m(-2) with bias under 500 W mercury lamp where the area is the cross-section of light irradiation. Structural analyses of the photocatalysts include atomic force microscopy, laser Raman spectroscopy and X-ray diffraction.