Abstract A detailed molecular-dynamics (MD) simulation study of the interfacial tension of the n-heptane+perfluoro-n-hexane (C6F14+C7H16) binary mixture is carried out at different temperatures and compositions for the vapour–liquid and liquid–liquid interfaces. The molecules are represented as chains of united-atom segments using previously reported force-field pure-component parameters with no further modification, while the unlike interaction energy between the alkyl and perfluoroalkyl segments is adjusted to describe the experimental data for the fluid phase equilibria. In the case of the vapour–liquid interface a horizontal inflection at a critical composition, sometimes referred to as aneotropy, is represented by our model as found in experimental work. Interfacial tensions for both the liquid-liquid and vapour-liquid interfaces are determined using the test-area method. A prerequisite of the determination of the interfacial tension by direct MD simulation involves studies of the fluid phase equilibria (vapour–liquid (VLE) and liquid–liquid equilibria (LLE)). A unique unlike energetic interaction parameter is adjusted to improve the prediction of the LLE and used in all the other simulations. The resulting description of the fluid phase behaviour is compared with that calculated using an equation of state (SAFT), and with experimental data where available; good agreement is observed.