In the context of nucleation, bubble formation and their growth within polymer melts interfacial tension (IFT) plays an important role especially for understanding the complete foaming procedure and for taking influence on the resulting polymer foam characteristics. Material and system properties like the range of viscosity and IFT, mass transfer phenomena and chemical composition affect the choice of experimental methods ford determining IFT. The PD technique has shown to be applicable to high experimental pressures and temperatures as well as relatively high liquid drop viscosity. In the presented investigation, different materials are applied in order to observe various effects that may take influence on the IFT of a polymer in condensed or supercritical fluid atmosphere. Liquid viscosity which varies with chain length and amount of dissolved gas does not take influence on the experimental result. The phase behaviour which also depends on molecular weight dominates the course of IFT at changing temperature and pressure. On the contrary to its monomer, complete mixing of polystyrene with compressed carbon dioxide is out of reach and IFT does not disappear within the investigated pressure range of up to 25 MPa. Based on a high number of experimental investigations, fluid mixtures are classified due to their mutual solubility in order to obtain qualitative information on IFT. (C) 2002 Elsevier Science B.V. All rights reserved.