The effects of air flow rate, oxygen partial pressure, aeration disk size and porosity, liquid height, and surface-active agents on aeration were studied in two unstirred towers or column-type vessels of different diameters using the sodium sulphite oxidation method. The coefficient of oxygen transfer increased with air flow rate up to the “flooding” point. The air flow rate required to cause “flooding” was increased by increasing the area of the aeration disk relative to the cross-sectional area of the tower. Within limits, aeration was improved by using disks of finer porosity. Under most conditions the coefficient of oxygen transfer was much higher in the small tower. The coefficient of transfer was not affected by liquid height in the tower, but efficiency of absorption increased with height. Aeration was adversely affected by all surface active agents tested.