Proper design and operation of separation processes are highly dependent on phase equilibrium calculations. The common method to treat polar/nonpolar systems, and also mixtures of normal compounds where the sizes are significantly different is to use binary interaction parameters. This paper presents a method to calculate mixture binary interaction parameters using closed cup flash point measurements. The measured and literature closed cup flash points of different binary mixtures (ethanol + n-octane, I-propanol + n-octane, 1-propanol + n-decane, n-octane + n-decane, 1-propanol + water, 1-butanol + n-octane, 2-butanol + n-octane, and water + propionic acid) are used to calculate binary interaction parameters. These binary interaction parameters together with common activity models are then used to calculate vapor-liquid equilibrium compositions of different binary mixtures considered in this study The validity of the proposed method was successfully approved by comparing the calculated equilibrium compositions with experimental data. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.