Transportation sources have created a major hydrocarbon pollution problem in the ambient air of Tehran. The authors used a Carbotrap tube to determine volatile organic compounds in air. Such compounds can be desorbed thermally and analyzed with gas chromatography-mass spectrometry. Samples were obtained from 8 sites in Tehran at which traffic flow varied between 500 and 2,500 vehicles/hr. A total of 54 hydrocarbons were identified in the ambient air of Tehran, and the average measured concentrations of benzene, toluene, m- and p-xylene, ethyl benzene, and o-xylene were 127.6 microg/m3, 201.1 microg/m3, 110.7 microg/m3, 58.1 microg/m3, and 57.6 microg/m3, respectively (standard deviation = 3.8-51.7 microg/m3). Emissions of individual pollutants in south Tehran exceeded those in north Tehran, and these emissions were higher during the afternoon than during the morning. The geographical parameters and the photochemical reaction also played important roles in the pollution conditions.