The essentials of the QCHB (quasi-chemical hydrogen-bonding) equation-of-state model are presented along with some applications for calculations of phase equilibria and interfacial properties of fluids and their mixtures. This is a model applicable to non-polar systems as well as to highly non-ideal systems with strong specific interactions, to systems of small molecules as well as to macromolecules, including polydisperse polymers, glasses, and gels, to liquids as well as to vapours including supercritical systems, to homogeneous as well as to inhomogeneous systems. A quasi-thermodynamic approach of inhomogeneous systems is used for modeling the fluid-fluid interface. Consistent expressions for the interfacial tension and interfacial profiles for various properties are presented. A satisfactory agreement is obtained between experimental and calculated surface tensions. Extension of the approach to mixtures is examined along with the associated problems for the numerical calculations of the interfacial profiles. A new equation is derived for the chemical potentials in the interfacial region, which facilitates very much the calculation of the composition profiles across the interface. The relation of the model with the COSMO-RS approach is also discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.