Essential oils containing aromatic compounds can affect air quality when used indoors. Five typical and popular essential oils-rose, lemon, rosemary, tea tree and lavender-were investigated in terms of composition. thermal characteristics, volatile organic compound (VOC) constituents and emission factors,. The activation energy was 6.3-8.6 kcal mol(-1), the reaction order was in the range of 0.6-0.8 and the frequency factor was 0.01-0.24 min(-1). Toluene, 1,2,3-trimethylbenzene,1,2,4-trimethylbenzene, n-undecane, p-diethylbenzene and m-diethylbenzene were the predominant VOCs of evaporating gas of essential oils at 40 degrees C. In addition, n-undecane, p-diethylbenzene, 1,2,4-trimethylbenzene, m-diethylbenzene, and 1,2,3-trimethylbenzene revealed high emission factors during the thermogravimetric (TG) analysis procedures. The sequence of the emission factors of 52 VOCs (137-173 mg g(-1)) was rose aroximate to rosemary > tea tree approximate to lemon approximate to lavender. The VOC group fraction of the emission factor of aromatics was 62-78%, paraffins were 21-37% and olefins were less than 1.5% during the TG process. Some unhealthy VOCs such as benzene and toluene were measured at low temperature; they reveal the potential effect on indoor air quality and human health. (C) 2009 Elsevier Ltd. All rights reserved.