Investigations on phase behaviour and interfacial phenomena in two and three phase fluid systems at pressures up to 250 MPa are presented. The interfacial tension, which is decisive for the energy input for dispersing one fluid phase in the other, was studied as a function of pressure and composition. The effect of pressure on the interfacial tension is generally found to depend on phase behaviour. Partly miscible systems tend to show decreasing interfacial tension as a function of pressure whereas inmiscible liquid-liquid dispersions are hardly affected. In this case, an additional gas phase being mixed with the adjacent liquid phases lowers the interfacial tension to a high extent. At pressures above 100 MPa it becomes increasingly difficult to measure the interfacial tension since the density difference may decrease to extremely low values and pressure induced crystallization may inhibit visualization of the interface. The aim of the presented investigation is to describe interfacial phenomena occurring in liquid-liquid-fluid systems at elevated pressures and quantify the effect of carbon dioxide as a fluid being partly miscible in a number of liquid phases on the interfacial tension. (C) 2005 Elsevier B.V. All rights reserved.