To improve the performance of a diffused aeration system, a series of aeration studies using various diffusers was initiated. At this stage, 35 unsteady-state reaeration test runs in a 1500-L aeration tank with coarse-bubble diffusers have been performed under varying diffused airflow rates (3.04-8.34 m(3)/h), water temperatures (17.7-34.7 degreesC), and water depths (0.6-1.8m). The reaeration data were analyzed by the traditional ASCE model and a two-zone model. The experimental results show that the volumetric mass-transfer coefficients of the ASCE model and of the surface reaeration zone increase with the airflow rate and temperature, but decrease with the water depth. The volumetric mass-transfer coefficient of the gas-bubble zone also increases with the airflow rate and water temperature, but is independent of the water depth. The saturation DO concentrations and the volumetric oxygen transfer rates under varying aeration conditions can be predicted by the two-zone model satisfactorily.