Doping TiO 2 photocatalysts with foreign ions has been deemed an effective method to enhance visible light absorption, and thus increase their photocatalytic performance. Herein, we report that Nb-doped TiO 2 porous microspheres prepared by ultrasonic spray pyrolysis of peroxide precursor solution are yellow colored, and the yellow coloration becomes increasingly conspicuous with increasing Nb dopant concentration.Comprehensive spectral analyses show that both surface peroxo species and bulk Ti 3+ are introduced into TiO 2 microsphere samples together by charge compensation with Nb 5+ dopant, and are responsible for the coloration of TiO 2 . The Nb-doped microspheres show higher photocatalytic rates than undoped TiO 2 for the degradation of gaseous acetaldehyde under visible irradiation, but slower rates under ultraviolet light. Moreover, the photocatalytic mineralization rates of acetaldehyde to CO 2 are lowered with Nb-doping under both visible and UV irradiation. Correlation between the results of surface photovoltage spectroscopic (SPS) characterizations and photocatalytic tests suggests that surface peroxo states are relevant to the visible light stimulated charge separation and photocatalytic reactions, albeit holes trapped in these states have lower reactivity than in valence band. On the other hand, the enhanced photoluminescence in the near-infrared region, reduced SPS response in the UV region as well as photochromic phenomena during photocatalytic process indicated that Ti 3+ defects serve as charge carrier recombination centers and display adverse effect to photocatalytic activity of Nb-doped TiO 2 , especially under UV irradiation.