Surface aeration experiments were conducted in three types of geometrically similar rectangular tanks of aspect ratios, i.e. length to width ratio (L/W) of 1, 1.5 and 2 in order to study the effect of aspect ratio on the oxygen transfer process. It is observed that for geometrically similar tanks, oxygen transfer coefficient (KLa20) is uniquely related to the mean velocity gradient (G). It has been found that at constant mean velocity gradient (G), KLa20 is high in lower aspect ratio tanks and it decreases as the aspect ratio increases. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.