An experimental study was made of phase inversion in an oil-water flow through a horizontal pipe loop. The experiments started with the flow of a single liquid through the pipe loop; thereafter the second liquid was gradually added (using different injectors and different injection flow rates) until inversion took place. It was found that in this way the point of inversion could be postponed to high values of the dispersed phase volume fraction (> 0.8). Samples were taken from the flowing mixture and inspected with the aid of a microscope. Multiple drops consisting of oil droplets in water drops were observed, but multiple drops consisting of water droplets in oil drops were never found. The significance of these observations for the phase inversion mechanism is discussed. (c) 2006 Elsevier Ltd. All rights reserved.