Multiphase flow occurs in many applications in the petroleum industry. Knowing how this phenomena behaves in different conditions and determining how it will impact the overall characteristics of the process is essential for project and optimization purposes. The thermophysical properties of the fluid have great influence on the multiphase flow. Particularly, the density of the gas phase will deeply impact on the flow pattern and in the pressure loss along the pipeline. In deep water offshore gas and oil production the gas phase has a high density, being of the same magnitude order as the liquid phases. In this work, the influence of the gas density on horizontal two-phase slug flow will be evaluated. In order to reproduce conditions similar to those found in the deep-sea production field, mineral oil and sulfur hexafluoride (SF₆) were applied. SF₆ was chosen due its high densities in lower pressures compared to other gases. Experiments were performed by using a 51-mm I.D., 500-mm long rocking-flow cell that was pressurized up to 35 bar at temperatures between 5°C and 45°C. The gas density ranged from 6 kg/m^³ to 388 kg/m^³, resulting in liquid-gas density ratios from 142 to 2.3. It was observed a reduction of the elongated bubble length with the increase of gas density. A transition from a smooth surface to a wavy one as the density increases was also noticed.