Paper reports on an experimental study of the air-entraining roller zone of hydraulic jumps in open channels. Hot-film anemometry techniques were used, involving instant digitizing of analog signals and delayed electronic processing of the tape-recorded data. The two phases (air versus water) were identified by a signal threshold. Results are presented on void ratio and average size and statistical size distribution of the entrained air bubbles for jumps with and without fully developed inflow. Findings confirm that the state of flow development of the supercritical stream entering the jump has a significant bearing on the resulting flow configuration within the jump body and thus, on the air-entraining capacity of the hydraulic jumps.