Although the hydraulic jump has been investigated experimentally for nearly two centuries, little information is known of the air-water flow properties in the shear region. New experiments were performed in a horizontal channel with partially-developed inflow conditions. Distributions of air concentration, mean air-water velocity and bubble frequency were recorded and presented herein. The results indicate an advective diffusion of air in the shear layer. The velocity profiles have a similar shape as wall jet flows but different quantitative parameters must be introduced. The relationship between air content and bubble frequency has a parabolic shape which is not yet understood but was observed previously in open channel flows. (C) 2000 Elsevier Science Ltd. All rights reserved.