The vapour pressures of binary (cyclopentanone + 1-chlorobutane, +1,3-dichloropropane, and +1,4-dichlorobutane) mixtures, were measured at the temperatures of (298.15, 308.15, and 318.15) K. The vapour pressures vs. liquid phase composition data have been used to calculate the excess molar Gibbs free energies G(E) of the investigated systems, using Barker's method. Redlich-Kister, Wilson and NRTL equations, taking into account the vapor phase imperfection in terms of the second virial coefficient, have represented the G(E) values. No significant difference between G(E) values obtained with these equations has been observed. (c) 2007 Elsevier Ltd. All rights reserved.