Flip buckets are terminal spillway elements, generating free jets for energy dissipation. Deflectors are an alternative to flip buckets with constructional ease yet of similar hydraulic features. A first experimental study published at the 33rd IAHR Congress, Vancouver, investigated deflectorgenerated jets in terms of their air concentration characteristics. For jet impact onto a movable bed, the scour potential reduces as the jet air content increases. It was demonstrated that the jet air concentration development is a function of its relative black-water core length, depending on the approach flow parameters and the deflector geometry. Jet-generating structures are located at the end of spillways, where high-speed flow with considerable air transport prevails, affecting the jet parameters, such that additional experiments were conducted involving pre-aerated approach flow. The effect of pre-aeration on the air concentration development of deflector-generated jets is discussed and compared with un-aerated approach flow.