Experiments were conducted to study the performance of circular and square tank surface aerators on the oxygen transfer coefficient and to a limited extent on power requirements. All the tanks are geometrically similar except for their shapes. They consist of a rotor of diameter, D, fixed with six flat blades rotating with a speed, N, in pure waters of viscosity, υ, at room temperature. A simulation equation to predict the oxygen transfer coefficient, k = KLa20(υ/g2)1/3 for any given dynamic parameter governing the theoretical power per unit volume, X = N3D2/(g4/3υ1/3) was developed for circular tank aerators. The data on square tank aerators support another such simulation equation developed earlier for square tanks. A comparison of results, while re-aerating the same volume of water in both the shapes of tanks, leads to the interesting conclusion that for a given rotor speed the oxygen transfer rate is substantially more in square tanks than in circular tanks; whereas for a given effective input power to the rotor the oxygen transfer rate is more in circular tanks than in square tanks. This suggests that square tanks are preferred to circular tanks to raise the oxygen concentrations at a faster rate, whereas the circular tanks are advantageous as far as power requirements are concerned.