Acoustic emission measurement is a possible method,for non-invasive, real-time process monitoring. The monitoring of acoustic emissions from a laboratory scale plunging jet at low velocities has been shown to allow identification of the onset of air entrainment, differentiate between certain types of hydrodynamic regime. The acoustic technique has been used to measure the variation of average bubble size and bubble size distribution with jet flow rate and nozzle height. NaCl and xanthan gum solutions have been used to demonstrate the effects of coalescence repression and viscosity respectively on variation of bubble size with jet flow rate and nozzle height. (C) 2003 Elsevier B.V. All rights reserved.