Slug flow is a very common occurrence in gas-liquid two-phase flow. Usually, it is an unfavorable flow pattern due to its unsteady nature, intermittency and high-pressure drop. For the calculation of pressure drop and void fraction, the normal approach is to separate the slug unit into two zones, a liquid slug zone and a film or gas zone. The pressure drop is calculated using slug flow models that were developed on the basis of a solution of the momentum and continuity equations for these two zones. When applying these models for downward flow, conditions can be encountered where no solutions exist. In this work, we closely examine these conditions and discuss their physical meaning, This includes a detailed description of the slug dissipation process in a downhill section and the calculation of slug dissipation distance. (C) 2000 Elsevier Science Ltd. All rights reserved.