The effect of liquid properties on flooding in small diameter vertical tubes is studied for various liquids with the aim to contribute to the interpretation of flooding mechanisms in such geometries. Data on free falling film characteristics for the various liquids used are acquired using a photographic technique from which mean layer thickness and its statistical quantities are calculated. The experimental data confirm previous observations that in almost all cases the dominant mechanism is wave growth and upward dragging by the gas and that consequently the onset of flooding is strongly affected by the liquid film structure. The results also confirm the influence of the liquid properties on the interfacial wave evolution and film characteristics. New correlations based on dimensionless groups for the prediction of flooding in narrow passages are proposed and found to be in good agreement with available data. (c) 2005 Elsevier Ltd. All rights reserved.