A backlit digital imaging technique was used to obtain images of bubbles within the liquid film of adiabatic air-water horizontal annular flow. A digital image processing algorithm that isolated bubble contours from other flow features was implemented. The bubble entrainment statistics showed that the bubble size distribution within the liquid film was exponential, and the parameters of the distribution were observed to be dependent on air flow rate and essentially independent of liquid flow rate. The bubble data, together with fluorescent imaging of waves on the liquid film, indicated that gas entrainment in the film was primarily controlled by air flow rate and wave behavior. This was confirmed by the existence of a consistent Weber number based on the maximum observed bubble diameter and disturbance wave velocity. (C) 2004 Elsevier Ltd. All rights reserved.