Aeration plays an important role in the activated sludge process; it must supply enough oxygen to maintain the metabolic reactions of microorganisms and provide sufficient mixing in the aeration tank. During aeration, oxygen is transferred to the aeration tank while volatile organic compounds (VOCs) are stripped from the tank and cause air-pollution problems. In this study, a series of batch VOC emission tests were performed in a 500-L tank equipped with coarse-bubble diffusers at 1.64-3.28 m(3)/h diffused air-flow rates and 297-299 K water temperatures in the absence and presence of commercial surfactant. The unsteady-state dissolved concentrations of p-xylene were measured during the tests and compared with the results predicted by both the American Society of Civil Engineers (ASCE)-based model and the two-zone model. According to the experimental results, the VOC emission rate increased with increasing airflow rate and decreasing surfactant concentration. The results also confirmed that the two-zone model could give a better prediction of the VOC emission rates while the ASCE-based model underestimated the VOC emission rates.