a b s t r a c t Bi 2 WO 6 with different Bi 3+ ion concentrations (0–30 mol%) is incorporated with the ZnWO 4 photocatalyst to improve the photocatalytic efficiency by forming a ZnWO 4 /Bi 2 WO 6 composite photocatalyst with hier-archical heterostructure via a one-step hydrothermal method. X-ray diffraction and Raman spectroscopy analyses confirm the presence of ZnWO 4 and Bi 2 WO 6 as main phases in the composite photocatalyst. Scanning electron microscopy and transmission electron microscopy observations reveal that the rice-and plate-like Bi 2 WO 6 nanoparticles were either separate or attached to the surface of quasi-spherical ZnWO 4 particles. A gradual increase in the Bi 2 WO 6 content of the composite photocatalyst results in a monotonic shift of the absorption edge from ca. 355 nm to longer wavelengths up to ca. 450 nm. The pho-tocatalytic performance of the ZnWO 4 /Bi 2 WO 6 composite photocatalyst was evaluated by investigating the degradation of gaseous acetaldehyde (AcH) under UV light irradiation. Only the composite photo-catalyst synthesized with 30 mol% Bi 3+ exhibits higher photocatalytic activity under UV light irradiation compared with both individual ZnWO 4 and Bi 2 WO 6 , and with a mechanically mixed ZnWO 4 /Bi 2 WO 6 composite photocatalyst. The enhanced photocatalytic activity is attributed to the n-n isotype junction formed between the two semiconductors and the charge separation of each semiconductor.