One of the more promising applications of ultraviolet photocatalytic oxidation (UVPCO) technology is air purification for improved indoor air quality (IAQ). Successful implementation has been hindered by lack of understanding of the behavior of UVPCO systems in real, constantly evolving ambient air environments. The focus of most published research is on the demonstration of the rate of removal of various single compounds in a laboratory experiment, often at high concentrations relative to those encountered in ambient air. UVPCO products have been released worldwide with wildly varying claims of effectiveness and lifetime, but limited data is available relative to the long term effectiveness of this promising technology. This study presents results from laboratory photocatalyst deactivation studies, and field testing of prototype reactors in two office building locations in CT, USA. The goal was both to investigate deactivation by silicon-containing volatile and semi-volatile organic compounds (VOCs and SVOCs) and to determine the durability of the photocatalyst in situ in an office environment. Ambient air quality was monitored by total VOC sensors, and relative humidity and temperature were documented. Limited air analysis was performed. Photocatalysts employed in the testing were either Degussa P25 or a 3% WO3 coated P25. The results indicate that rapid catalyst deactivation is observed in photocatalytic air purifiers deployed in ambient office air. (C) 2010 Elsevier B.V. All rights reserved.