This paper looks into the effect of the pressure on the transition from stratified to slug gas-liquid flow in a horizontal pipeline. A systematic campaign of experiments was carried out at different pressures using the 4-in (108.2 mm ID) loop of the Multiphase Flow Loop test facility at National University of Singapore. Air and water are used as the gas and liquid phases, respectively. The air pressure in the flow loop was varied in the range 0-10 Barg. By changing the pressure, the gas density is changed without affecting other properties of the fluids. The liquid height in the pipe was measured using gamma ray densitometer, which is particularly suitable when steel pipes are used to handle the pressure in the flow loop. The time series of the liquid height was recorded using LabVIEW, and further data analysis was performed offline. In the range of 0-3 bar, the flow was visualized through a replaceable transparent section and a high speed camera. Given a flow condition of gas and liquid superficial velocities at the test section of the flow loop, pressure effects in terms of the flow pattern transition have been identified. The stratified-slug boundary moves up in the flow pattern map with increase in the pressure. In addition, it was found that, abrupt changes in the average liquid holdup when the pressure changed are linked to the transition. This effect has been analyzed and the data herein reported are useful to validate models to predict the flow behavior at much higher pressures. (C) 2016 Elsevier Ltd. All rights reserved.