Isothermal vapor-liquid equilibria at 333.15 K, 343.15 K and 353.15 K for three binary mixtures of o-xylene, m-xylene and p-xylene individually mixed with N-methylformamide (NMF), have been obtained at pressures ranged from 0 kPa to 101.3 kPa over the whole composition range. The Wilson, NRTL and UNIQUAC activity coefficient models have been employed to correlate experimental pressures and liquid mole fractions. The non-ideal behavior of the vapor phase has been considered by using the Peng-Robinson equation of state in calculating the vapor mole fraction. Liquid and vapor densities were measured by using two vibrating tube densitometers. The excess molar volumes of the liquid phase were also determined. Three systems of o-xylene + NMF. m-xylene + NMF and p-xylene + NMF mixtures present large positive deviations from the ideal solution and belong to endothermic mixings because their excess Gibbs energies are positive. Temperature dependent intermolecular parameters in the NRTL model correlation were finally obtained in this study. (C) 2009 Elsevier B.V. All rights reserved.