Downwards co-current gas-liquid annular flows were studied experimentally and characterized. An advanced optical laser-based measurement technique, namely Planar Laser-Induced Fluorescence (PLIF), was used for the visualization of the annular flow over a range of liquid Reynolds numbers Re-L = 306-1 532 and gas Reynolds numbers Re-G = 0-84 600. Four distinct flow regimes, namely the 'dual-wave', 'thick ripple', 'disturbance wave' and 'regular wave' regimes, have been identified based on qualitative information and a consequent quantitative analysis that provided information on the film thickness, interface and wave statistics, and gas entrainment into the liquid film. The mean film thickness data are generally in good agreement with previous studies. Evidence suggests that the turbulent gas phase affects strongly the shape of the interface, and that the mechanism for gas entrainment into the liquid film is strongly reliant on the existence of large-amplitude waves. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.