Liquid-side mass-transfer coefficients (ha) were measured in surface aerators and self-inducing-type reactors. The measurements were made in 1.0- and 1.5-m-i.d. cylindrical and 8.2 x 8.2-m square tanks using physical absorption of oxygen. The effect of liquid submergence was investigated in detail. Further, the effects of the diameter of the tank, the ratio of the impeller diameter to the tank diameter, and the clearance of the impeller from the tank bottom were also studied. Three different impeller designs were investigated as surface aerators. Single and multiple impellers were employed in the case of self-inducing reactors. The power consumption was varied in the range of 30-1000 W/m(3). Suitable correlations were developed for both types of reactors.