In this paper the gradient theory is applied for modelling the interfacial tensions of planar interfaces in multicomponent systems, in which one or more associating components are present. For this purpose, the square gradient theory is combined with the Associating-Perturbed-Anisotropic-Chain Theory. The results have been compared with results obtained with the gradient theory combined with the Peng-Robinson equation of state. The interfacial tensions computed from the new model agree very well with experiments for systems composed of normal alcohols and water. Even the interfacial tensions of a ternary system composed of water, hexane, and ethanol can be described accurately. In order to improve the accuracy the influence parameter was a linear function of temperature. (C) 1998 Elsevier Science B.V. All rights reserved.