The scope of this study is to demonstrate how density functional theory as part of the statistical mechanics of inhomogeneous fluids can contribute to a better understanding of the internal structure and interfacial properties of vapor-liquid and liquid-liquid interfaces in multicomponent liquid mixtures. Especially fluid mixtures with liquid-liquid phase separation can serve as models for liquid-liquid extraction systems. The partial density profiles in such an interface describe molecular concentration gradients which may act as a driving force for mass transfer across this interface. In the application of the density functional theory to liquid-liquid interfaces, we analyzed the influence of different approximations within the theory on the calculation of interfacial structure and thermodynamic potential properties. Especially the use of a weighed density approximation allows us to take short-range local ordering into account. It results in oscillating partial densities or concentration profiles and it yields a highly structured grand canonical potential density across the interface which demonstrates how energetically inhomogeneous a liquid interface is. For a ternary model system, the interfacial tension is calculated, capillary wave contributions are added and their influence is discussed. (c) 2004 Elsevier B.V. All rights reserved.