In this study, the authors report on the potential application of hyperbranched polymers as surface tension modifiers in linear polymers or compatibilisers in polymer blends. The influence of hyperbranched polyesters (HBP) with different functional end groups on the surface tension of mixtures with an oligo(ester diol) was investigated. The surface tension of the pure components and of the mixtures was studied within a temperature range of 20°C up to 100°C. The results indicate that the surface tension of the pure HBPs strongly depends on the functionality of the end-groups. The modification of the hydroxyl end groups by short alkyl chains (methyl, tert-butyl) reduced the surface tension, depending on the degree of functionalisation. The surface tension of mixtures could be considerably decreased using 1 % of HBP that was partially substituted with short alkyl chains. In this case, the modified HBPs acted as surface active agents. On the molecular level, the enrichment of the modified HBP in the surface region was proven by XPS (x-ray photoelectron spectroscopy) measurements. Copyright OCCA 2005.