The aim of this study is to better understand and model atomisation phenomena occurring in a venturi metre under air/oil and air/water annular flows. First, the structure of the liquid film was observed using a high-speed video camera in the pipe section upstream of the venturi, and along the convergent section: the surface waves and regimes visualised were described and compared with the literature, which highlighted the influence of the liquid film physical properties. Some of these surface waves, corresponding to disturbance waves as described in the literature, seem to be at the origin of consequent atomisation. These disturbance waves were then characterised through calculation of their velocities and comparison with the literature: a calculation procedure was defined and showed the influence of parameters such as air superficial velocities, wave height, liquid viscosity, interfacial friction velocity and flow regimes. Another aspect of the atomisation modelling consists in modelling both the location of the beginning of atomisation and the atomisation rate inside the venturi. The video records showed that, contrary to what is often assumed in some studies, a consequent atomisation may start at the middle of the convergent section, depending on the surface waves occurring at that time. Finally, a correlation law for the atomisation rate is proposed, based on the literature and measurements on air/water and air/oil flows. (c) 2012 Elsevier Ltd. All rights reserved.