A unified mechanistic model for slug liquid holdup is developed based on a balance between the turbulent kinetic energy of the liquid phase and the surface free energy of dispersed spherical gas bubbles. The turbulent kinetic energy is estimated by use of the shear stress at the pipe wall and the momentum exchange (mixing term or acceleration term) between the liquid slug and the liquid film in a slug unit. The momentum exchange term varies significantly with pipe inclination and enables the model to give an accurate prediction of slug liquid holdup for the entire range of pipe inclination angle. The model has been compared with experimental data acquired at TUFFP for slug flows at all inclinations and good agreement has been observed. The model can also be used to predict the slug-dispersed bubble flow pattern transition boundary over the whole range of inclination angles. From comparison with previous experimental results, the model predictions are accurate for gas superficial velocities larger than 0.1 m/s. (C) 2002 Elsevier Science Ltd. All rights reserved.