A major problem in the production of natural gas is liquid loading, i.e. the accumulation of liquids at the bottom of a well at low reservoir pressures. To prevent liquid loading, surfactants are injected at the bottom of the well, which changes the multiphase flow in the well tubing such that a smaller gas velocity is required to transport the liquids to the surface. However, no predictive models for the effect of surfactants on gas-liquid pipe flows are available. We present results of systematic experiments on the effect of surfactants on air-water flow in vertical pipes. The surfactants lead to the creation of foam, which has a lower density than water and can be transported by the gas more easily, which leads to a smaller pressure gradient at small gas flow rates. This effect is independent on the type of surfactant used. Furthermore, from the results we obtained a relation between the thickness of the film at the wall of the pipe and the interfacial friction between the gas and the film. Using the knowledge obtained from the experiments, we developed a mechanistic model for air-water flow with surfactants. This model is able to capture the trends in the pressure gradient observed in the experiments. This work is sponsored by NAM, a Dutch subsidiary of Shell and ExxonMobil@en