Abstract: By reducing intermittency to periodicity, the very complex slug flow structure is simplified to an equivalent unit consisting of a liquid slug, one adjacent Taylor bubble and an aerated liquid film. A region with high local concentration of dispersed bubbles is formed in the front of the liquid slug, because the liquid film entrains gas penetrating the liquid slug. In the current paper, a hydrodynamic model is constructed for gas - liquid two - phase slug flow in vertical tubes, which considers the gas exchange between the Taylor bubble and its following liquid slug. It can predict the flow characteristics of this complex flow pattern. A detailed experimental investigation was carried out in air/water system. An EKTAPRO 1000 high speed motion analyzer and the optical probes were used as measuring instruments. The results predicted by this model are in better agreement with the available experimental data.