Oxygen mass transfer in drop structures plays important roles in natural waters such as cascade waterfalls and in wastewater treatment facilities such as sewer collection systems and equalization tanks. To assess the oxygen transfer rate in drop structures, the oxygen mass transfer process was studied in a tank with water recirculation and impingement. A series of un-steady-state reaeration experiments with varying water recirculation rates (4-12 L/min), varying drop heights (0.6-1.4 m), varying water depths (0.6-1.4 m), and varying water temperatures (18.7-29.5 degreesC) were performed, and the reaeration data were analyzed by a mass transfer model considering the oxygen transfer processes in the droplet and surface mass transfer zones. The experimental results showed that the oxygen transfer rate of the surface mass transfer zone increased with increasing water recirculation rate, or drop height, while the oxygen transfer rate of the droplet mass transfer zone increased with increasing water recirculation rate, drop height, or droplet average temperature. Semiempirical equations were used to correlate the parameters of the model as functions of water recirculation rate, drop height, water depth, and temperature.