Photocatalytic activities of amorphous and crystal bismuth tungstate (Bi2WO6)were investigated using oxidative decomposition of gaseous acetaldehyde under visible light irradiation (>400 nm). Here, for the first time, negligible photocatalytic activity of amorphous Bi2WO6 owing to the fast recombination of electron-hole pairs and the high quantum efficiency of Bi2WO6 crystallites under visible light were demonstrated by action spectrum analysis and time-resolved infrared absorption measurements. Crystallization of the amorphous phase provided a red shift of the photoabsorption edge and marked increase in the lifetime of photoexcited electrons, resulting in an increase of photocatalytic activity.