This paper deals with theoretical investigation of interfacial properties between two coexisting liquid or fluid phases in thermodynamical equilibrium. The Cahn-Hilliard gradient theory was combined with an activity coefficient model (Koningsveld-Kleintjens model) or with an equation of state (Sanchez-Lacombe lattice fluid model). Using an activity coefficient model, only one variable (concentration) changed passing the interface. The interfacial tension between demixed liquids was calculated for two systems (water + ethylene glycol isobutyl ether (EIB) and water + diethylene glycol diethyl ether(DDE)). Adjusting one parameter, the theory yields satisfactory estimates of interfacial tensions as a function of temperature. Applying an equation of state, two variables (density and concentration) change in the interface. In this case the theory gives the density profiles of both components. The calculated density profiles demonstrate the ability of the concept to predict preferential adsorption phenomena in binary systems. The theoretical approach was applied to three systems (aniline + cyclohexane, acetone + carbon disulfide, and stearic acid + propane). If the systems show a closed miscibility gap, the interfacial tension passes a maximum. A comparison of theoretical and experimental interfacial tensions of the system aniline + cyclohexane indicates that the theoretical concept is able to describe the experiment if two parameters were fitted.