A new type of flow structure has been observed in cocurrent adiabatic vertically downward air-water flow and is described here. Membranes, in the form of thin liquid films spanning the channel cross-section and horizontally dividing the gas core were visualised over a wide range of air and water flow rates. Videos recorded with a high-speed digital camera showed that these membranes formed by the coalescence of dispersed bubbles, from the interaction between large entrained bubbles and the annular liquid film, or from the shrinkage of the liquid slugs. The objectives of this paper are to describe the characteristics of these membranes, to present how they interact with other two-phase flow structures, and to investigate how they form and develop as they are convected along the annular liquid film by the air core. The influence of the air-water mixer, the test section geometry, and the operating conditions on the formation of these thin liquid membranes are also assessed. (C) 2013 Elsevier Ltd. All rights reserved.