Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices (aerators) introducing air in the layers close to the channel bottom in order to reduce cavitation damage. The performance of a spillway aerator are studied on steep spillway model with high velocities (from 4 m/s up to 15 m/s) and new air concentration and velocity measurements were performed. The data are presented and analysed developping new equations. The behaviour of the air demand relationship between the air discharge and the subpressure in the cavity beneath the nappe is analyzed. All these results provide a better understanding of the air entrainment processes above a spillway aerator and the quantity of air entrained can be obtained.