Various types of volatile organic compounds (VOCs) have been found to exist in different indoor environments. They usually exist in levels that are below recommended standards. However, the combined effect can be significant. Use of chemical filtration units in the market do not perform as effectively as the manufacturers claimed. One reason is that the testing conditions were usually different form the actual condition on sites and information on VOCs in the indoor environment is scare. The Hong Kong University and Engineering (HK) Ltd. On developing a new chemical based filtration unit. Phase one of our study was to identify and to quantify the VOCs on various office premises in HongKong using both GC/MS and PAS techniques. In this paper, measurement results on various office buildings , new renovated domestic buildings and office buildings were presented to charcterize the most abundant volatile organic compounds and their levels under various air conditioning conditions. Operation pattern of the air-conditioning system, ventilation rate, surrounding VOC levels and indoor characteristics all showed significant influence on the indoor VOC levels. Discussion of our testing device and study protocol is also included in this paper.