Abstract Two types of TiO 2 hydrosols (TOSO and HTO) were prepared from titanium sulfate (TiOSO 4) and metatitanic acid (H 2TiO 3) by a chemical precipitation–peptization method, respectively. The prepared hydrosols were characterized by means of X-ray diffraction, particle size distribution, scanning electron microscopy, UV–vis spectroscopy, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller and Barret–Joyner–Halender methods. The results showed that the TiO 2 hydrosols with an anatase crystal structure had smaller particle sizes, higher surface areas, larger pore volume, and higher transparence than Degussa P-25 suspension. The photocatalytic activity of the TiO 2 hydrosols was evaluated for formaldehyde degradation under UVA illumination in a gaseous phase. The results demonstrated that the photocatalytic activity with the catalyst loading of 2 mg cm −2 was ranked as an order of HTO > TOSO > P-25. The photocatalytic activity was further studied using the HTO catalyst under different experimental conditions. The results showed that catalyst loading, relative humidity, and initial concentration could influence the efficiency of HCHO photocatalytic degradation. It was found that a catalyst loading of more than 2 mg cm −2 and a relative humidity of 55% were two essential conditions for achieving the best performance under these experimental conditions. The repeated experiments indicated that the HTO catalyst was reasonably stable and could be repeatedly used for the HCHO oxidation under UVA irradiation. This investigation would be helpful to promote the application of TiO 2 photocatalytic technique for indoor air purification.