Abstract A study on the gas phase expansion and gas hold-up occurring in free bubbling vertical slug flow is reported. A slug flow simulator (SFS) supported by air–water experimental data was used for this purpose. The study was accomplished by implementing in the simulator the gas phase expansion along the column. Effects over bubble lengths and bubble velocities were considered. The flow in 6.5 and 20 m long columns with internal diameter of 0.032 m was simulated. Expansion of gas phase along the column is shown to slightly decrease the occurrence of bubble coalescence. Liquid free surface oscillations (due to bubble burst events and continuous inlet of liquid and gas in the column) were found to affect the expansion of the gas phase, especially for high gas flow rates. The gas phase expansion for different outlet column configurations was studied. The use of a high and large cross sectional tank (to dampen free surface oscillations) is shown not to assure a permanent expansion rate of the gas phase. Simulations with and without gas expansion along the column were compared for the computation of average flow parameters. Approximate approaches (with constant UG, corrected for the mid-column pressure) were found suitable for the prediction of the average slug length and gas hold-up. Those approaches are, however, inadequate for the computation of the average bubble length and velocity along the vertical coordinate of the column.