An experimental study of the phase inversion process in an oil-water flow through a pipe was carried out. Special attention was paid to the critical concentration of the dispersed phase at which phase inversion occurs and to the change in morphological structures during inversion. To that purpose two different types of experiments were performed: (1) continuous experiments during which the dispersed phase fraction was gradually increased and (2) direct experiments whereby oil and water were injected simultaneously into the pipe at a certain concentration. During the experiments detailed pictures were taken of the phase inversion process and simultaneously the electrical conductivity of the mixture was measured to determine which liquid formed the continuous phase and which the dispersed phase. Also the pressure gradient over several parts of the pipe was measured. For continuous experiments the critical concentration was found to depend on the injection phase volume fraction. The critical concentration was significantly higher for continuous experiments than for direct experiments. The change in morphological structures during phase inversion was the same for the two types of experiments. During inversion the concentration of drops of the (originally) dispersed phase becomes so high, that they coalesce at certain places in the flow field and form relatively large, rather complex, morphological structures. With a further increase in concentration of the (originally) dispersed phase these morphological structures grow in size and start to form the new continuous phase in which again complex structures are present, but this time consisting of the (originally) continuous phase. (c) 2008 Elsevier Ltd. All rights reserved.