The oxygen transfer rate, which is the most energy demanding unit process, is an essential factor in the operation of an activated sludge plant. Energy conservation effort has been directed at designing high efficiency and optimal air diffusers for use in activated sludge aeration tanks. This paper proposes a model for oxygen transfer rate in diffused air systems. The model parameters were found by means of nonlinear regression analysis, using plant data given in the literature. The proposed model may be useful for the preliminary design of diffused air aeration tanks.