We develop a method for treating an interface between coexisting fluid phases to study its structure and thermodynamical properties. Our approach includes calculated correlation functions. Approximations are intrinsically optimized. The method is successfully applied to the liquid vapour surface of a Lennard-Jones fluid, to the liquid–liquid interface of a demixing system and to the surface of a Stockmayer liquid towards vapour. In all cases we compare our results to simulations. The agreement with simulation results demonstrates the reliability of our approximations and shows that the technique we applied provides a powerful and robust method for studying inhomogeneous fluids.