We have developed the next generation of photocatalytic testing to observe the gas-phase photocatalytic reactions in situ. It is based on modified Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) unit equipped with the Smart Collector system. We have employed UV diode coupled with fiber optics to illuminate the sample while simultaneous collecting the diffuse reflectance spectra. This method was applied to study the NO2 photocatalytic conversion on nanosize gold modified TiO2 samples. We have successfully demonstrated that both gas phase and surface adsorbed species can be identified and used to compare activities of various catalysts. We have found that gold nanoparticles have significantly increased the activity of the catalysts for both gas phase and liquid phase oxidation, resulting in more than quadrupling the NO2 photocatalytic conversion in the gas phase. (C) 2010 Elsevier B.V. All rights reserved.