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Abstract: In storm waterways and at dam outlets, high-velocity supercritical flows are characterised by substantial air bubble entrainment. The entrainment of fine air bubbles and the strong turbulent mixing contribute both to the air-water transfer of volatile gases (e.g. oxygen, nitrogen, VOC). The paper describes new experimental data obtained in a 25-m long channel with a 4-degree slope. The analysis of the data provides new information on the air-water flow properties and on the distributions air-water interface area. Although the amount of entrained air is small (i.e. typically Cmean < 0.12), the specific air-water interface area can reach over 100 m2 per unit volume of air and water. The result are compared with an earlier prediction (CHANSON 1994) and confirm the significant contribution of air entrainment to air-water gas transfer in supercritical chute flows. Key words: air-water interface area, experimental measurement, open channel flow, air bubble entrainment, gas transfer. Nomenclature a = specific interface area (m-1) defined as the air-water surface area per unit volume of air and water; C = air concentration defined as the volume of air per unit volume of air and water; it is also called void fraction; Cmean = mean air concentration defined in term of Y90: (1- Cmean*Y90) = d; ch = chord length (m); chab = air bubble chord length (m); (chab)max = maximum chord length (m) detected during the scanning time t; (chab)NMS = mean bubble size (m) or the Number Mean Size defined as: (chab)NMS = i =