Nanosized ZnWO 4 photocatalysts were successfully synthesized via the sol-gel process in a temperature range of 450—800°C. The grain size, crystal size, and crystallinity of ZnWO 4 particles increased with the increase of calcination temperature and prolonging calcination time. The photocatalytic activity was measured for the degradation of an aqueous Rhodamine-B (RhB) solution and gaseous formaldehyde (FAD). With the increase of calcination temperature and time, the activities increased to a maximum and then decreased. ZnWO 4 photocatalyst prepared at 550°C for 10 h showed the highest activity, which is similar to the photocatalytic activity of P25TiO 2 for the degradation of gaseous FAD. High crystallinity, large surface area, and good dispersion are responsible for the high photocatalytic performance of the prepared ZnWO 4 .