An attempt is made to model slug flow inside square capillaries containing Newtonian liquids. In square micro-channels, slug flows proved to promote high rates of heat and mass transfer between the solid boundaries and the bulk flow. This is due to the combination of bubble induced secondary flows and the creation of very thin liquid films. There are many industrial applications for which it is necessary to predict this rate of transfer. The volume of fluid (VOF) method implemented in the commercial CFD package, FLUENT is used for this numerical study. A comprehensive description of such flow is obtained and a comparative study is conducted on the hydrodynamics of slug flow inside circular capillaries and their square counterparts. Computed values of the bubble velocity and diameter were in excellent agreement with published experimental measurements. The detailed velocity field around the bubble was also computed and compared favourably with those experimental results reported in literature. Based on the hydrodynamic study explanation of mass and heat augmentation due to slug flow is drawn. (c) 2005 Elsevier Ltd. All rights reserved.