Adsorption/desorption behavior of polycyclic aromatic hydrocabons (PAHs) in the atmosphere has been studied. Test pieces (4.7 cm diameter) were prepared by cutting out the glass fiber filter on which airborne particulates were collected. Air containing PAHs at various concentrations passed through the test pieces at 15 / /min for 48 h at ambient tempe rature (average temperature 9.1°C), and the amounts of PAHs in the test pieces were determined. In the case of PAHs which have high vapor pressure (10-5≈10-6 mmHg) such as fluoranthene. and pyrene, the amounts of PAHs in the test pieces increased linearly with increasing the concentrations of PAHs in the air passed and decreased when PAH-free air was passed. The adsorption of PAHs on particulates seemed to be reversible, and Langumuir adsorption isotherm in the low coverage would be applicable to the adsorption/desorption process of PAHs between vapor phase and particulate phase in the ambient air.