The behaviour of interfaces in fluid systems is of fundamental and practical importance. Interfacial tensions in fluid mixtures are often required for the design of chemical process equipment. In oil reservoirs, the surface/interfacial tension is held responsible for the migration of hydrocarbons and, therefore, the recovery from these reservoirs. A theory, which is of particular significance for computation of interfacial tensions is the gradient theory (GT), originally introduced by Rayleigh (1892) and van der Waals (1894). The required inputs of gradient theory are the Helmholtz free energy density of the homogeneous fluid and the so-called influence parameters, Cij of the inhomogeneous system. This influence parameter is related to the direct correlation function of the homogeneous fluid. This result means that all the important inputs of gradient theory can be derived from the properties of the homogeneous fluid. The relation for the Helmholtz free energy density was derived from the associated perturbed anisotropic chain theory (APACT). This theory was designed to treat mixtures that associate through hydrogen-bonding or through the interaction between dipolar or quadrupolar molecules. One disadvantage of using APACT is that it overpredicts the critical pressure as well as the critical temperature for hydrogen-bonding, polar or non-polar fluids. This unfortunate effect also occurs in mixtures, as was shown for systems containing carbon-dioxide. To test the model, the interfacial tensions of water, linear alcohols from methanol up to 1-decanol, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-decane, carbon-dioxide, benzene, and their mixtures were calculated. The interfacial tension of the pure compounds can be accurately described when the influence parameter, fitted to experimental interfacial tension data, is a linear function of temperature. The ability to predict interfacial tension of mixtures was studied for systems, such as water/n-alkanes, n-alcohols/n-alkanes, n-alcohols/n-alcohols, water/n-alcohols, carbon­dioxide/n-alkanes, benzene/n-alcohols and benzene/n-hexane, water/methanol/1-propanol and carbon-dioxide/n-butane/n-decane. It was shown that the gradient theory in cooperation with the APACT equation of state can predict interfacial tension of liquid-vapor and liquid-liquid interfaces, of the studied mixtures, reasonably well. Predictions can even be approved by using a binary interaction parameter, kii which corrects the attractive part of APACT. In order to calculate interfacial tensions, the density profiles of all the components present in the mixture, must be obtained. A remarkable effect was observed in the obtained density profiles of 1-propanol/n-heptane. The increasing density, along the interfacial zone, of 1-propanol causes a decrease in the concentration of n-heptane. This effect was not unique for this system and occurs in almost every binary system with n-alcohols and an alkane or a n-alcohol as second component. A possible orientation of the alcohol molecules in the interface can cause this effect, resulting in a decrease of the n-heptane density.