Research was conducted to characterize stream reaeration capacity in terms of the stream hydraulic properties and to develop procedures for evaluating the effects of pollutants on reaeration. A gaseous tracer procedure was used in the field studies for measurement of reaeration. Not only have these studies incorporated a wide range of hydraulic features, but also, the range of BOD's and temperatures encountered was large. Tests of observed vs. predicted values of K(2) have shown that none of the available models is capable of providing dependable predictions of stream reaeration capacity, especially under highly turbulent flow conditions. A new model was derived that explains the reaeration capacity of a stream in terms of the rate of energy dissipation and furthermore, provides dependable predictions.