Ambient measurements of 108 volatile organic compounds (VOCs), including alkanes, alkenes, aromatics, and halogenated hydrocarbons, were conducted from 2002 to 2003 at six sites in Beijing city. The mean mass concentration of total VOCs was 132.6 +/- 52.2 microg/m3, with alkanes, aromatics, and alkenes accounting for 35%, 22%, and 17%, respectively. The concentrations of most VOC species showed a seasonal pattern, with higher values in November, mildly lower in March, and much lower in July. In winter and spring, apparent diurnal variations of reactive compounds such as 1,3-butadiene and isoprene were observed, whereas those were not distinct in summer. The propylene equivalent concentration was used to evaluate the contribution of individual VOCs in ozone formation. Reactive olefins from anthropogenic emissions dominated the reactions with OH at each season. In summer, isoprene became the largest contributor, followed by 1-butene and propene. The source profiles in Beijing, including vehicle exhaust, gasoline vapor, painting operations, and asphalt pavement, were investigated. Based on the measurement of source profiles and ambient concentrations of VOCs in Beijing, chemical mass balance receptor model was applied to estimate contributions of several potential VOCs sources in Beijing. The results indicated that vehicle exhaust contributed on average 57.7%, followed by painting operations, gasoline vapor, and liquefied petroleum gas (LPG) at 12.4%, 11.3%, and 5.8%, respectively.