The Group Contribution Equation of State (GC-EOS) is applied to model high-pressure phase equilibria in mixtures of fatty oils and acid alkyl esters with supercritical fluids. The fatty oil is represented by a pseudo-triacylglyceride with the same molecular weight and degree of unsaturation of the natural oil. The critical hard sphere diameter of the oil, i.e. the size-related parameter, is determined from the correlation of infinite dilution activity coefficients of n-alkanes in high molecular weight triacylglicerides and hydrocarbons. Good predictions of vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibria in mixtures of supercritical solvents (propane, ethane and CO2) with pure triacylglycerides, natural vegetable oils and fatty acid alkyl esters are obtained. These results show the capability of the GC-EOS model to describe the phase equilibria of size-a symmetric mixtures and its potential for modeling supercritical processes involving oils and derivatives. (C) 2002 Elsevier Science B.V. All rights reserved.