The quantity of the outdoor air brought into the building can have a direct negative effect on the energy cost of ventilation and the climate. There is cost to heat, cool, humidify or dehumidify the outdoor air depending on the location and season. This leads to a balancing act between indoor air quality (IAQ) and energy cost. Ultraviolet photocatalytic oxidation (UV-PCO) has been acclaimed as one of these innovative technologies for purifying indoor air by decomposition of pollutants, especially volatile organic compounds (VOCs). Although numerous studies have been carried out in this field, UVPCO performance under real life applications is still questionable. Experimental conditions of most previous works focused on ppm level of VOCs in ideal bench-top-scale reactor with low flow-rate and high residence time conditions. On the other hand, limited research has been devoted to investigate generation of the UV-PCO toxic by-products and this issue is one of the main drawbacks and obstacles in its wide applications and design of immune buildings. This paper represents the outcomes of an experimental study which was performed to investigate the generation of the by-products under single pass through duct system. By-products in photodegradation of n-hexane as a result of ozone producing VUV lamps with 254nm+185nm wavelength radiations have been compared with UVC lamps with 254nm wavelength radiation. Two types of catalyst media were used to investigate the feasibility of UV-PCO technology. Removal efficiency and by-products identification results are presented and discussed.