A mathematical model was developed to predict the air concentration profiles in the deaeration zone below a spillway aerator by solving the conservation equations of mass and momentum. The turbulent diffusion coefficient was calculated by ak-ε model and the air-water mixture was treated as a stratified flow. To test the mathematical model a series of experiments was conducted. Air was injected from the bed of a flow channel and air concentration profiles were measured at various downstream sections. From the comparison of the computer predictions with the experimental data it was concluded that the present analytical approach was satisfactory for practical purposes and showed promise for further development.