The flow of two immiscible liquids and the influence of an additional inserted gas phase in horizontal pipes is investigated. The experiments are carried out in a transparent horizontal pipe with an inner diameter of 59 mm and a total length of 48 m. Experimental results are presented for the flow regimes of the two phase and three phase flow of oil, water and gas mixtures. The effect of phase inversion on the pressure drop is measured. The experimental results obtained for the three phase flow of oil, water and air indicate that drag reduction is possible by injecting gas in laminar flowing mixtures of oil and water. In the aerated slug flow regime of oil, water and air a water dominated and an oil dominated flow system can be distinguished. The pressure drop of the three phase flow system is of the magnitude as the pressure drop of the two phase flow of gas and the dominating liquid phase.