Different flow patterns for lube oilwater and for kerosene-water downflow through a vertical glass tube have been analyzed with the help of flow visualization. Core-annular flow is the dominant flow regime, with oil forming the core, and water is forming the wall film. When the velocities are increased, transition to slug flow and transition to dispersed flow are found. The waves found during the transition to slug flow depend on oil viscosity: axisymmetric bamboo waves are seen in kerosene-water downflow and the waves are asymmetric in case of lube oilwater flow where they have a cork-screw shape. Based on the experimental observations, simple mathematical models have been proposed for predicting the flow pattern transition curves. (C) 2011 American Institute of Chemical Engineers AIChE J, 2012