A mixing rule based on two-fluid theory is introduced to consider the non-random mixing of spices in the calculation of the properties of mixtures when an equation of state for chain-like molecules is adopted. The equation is expressed as the sum of a reference term, a perturbation term and an association term. For systems of non-associated spices, the last term is vanished. The mixing rule is used for the second term. The method can be used to correlate vapor-liquid equilibria with only one adjustable parameter. It was found that using two-fluid theory could give better results than that using one-fluid theory for asymmetrical systems or for systems with only self-association. (C) 2003 Elsevier B.V. All rights reserved.