The present study evaluated visible-light photocatalysis, applying an annular reactor coated with unmodified or nitrogen (N)-doped titanium dioxide (TiO2), to cleanse gaseous volatile organic compounds (VOCs) at indoor levels. The surface chemistry investigation of N-cloped TiO2 suggested that there was no significant residual of sulfate ions or urea species on the surface of the N-cloped TiO2. Under visible-light irradiation, the photocatalytic technique using N-doped TiO2 was much superior to that for unmodified TiO2 for the degradation of VOCs. Moreover. the degradation efficiency by a reactor coated with N-cloped TiO2 was well above 90% for four target compounds (ethyl benzene, o,m,p-xylenes), suggesting that this photocatalytic system can be effectively employed to cleanse these pollutants at indoor air quality (IAQ) levels. The degradation efficiency of all target compounds increased as the stream flow rate (SFR) decreased. For most target compounds. a reactor with a lower hydraulic diameter (HD) exhibited elevated degradation efficiency. The result on humidity effect suggested that the N-cloped photocatalyst could be employed effectively to remove four target compounds (ethyl benzene, o,m,p-xylenes) under conditions of less humidified environments, including a typical indoor comfort range (50-60%). Consequently, it is suggested that with appropriate photocatalytic conditions, a visible-light-assisted N-cloped photocatalytic system is clearly an important tool for improving IAQ. (C) 2008 Elsevier B.V. All rights reserved.