The vapour pressures of pure components and two binary mixtures of (cyclopentanone + 1,2-dichloroethane and + l,l,l-trichloroethane) were measured at temperatures between 298.15 K and 313.15 K. The vapour pressures vs. liquid phase composition data have been used to calculate the activity coefficients and the excess molar Gibbs free energies G(m)(E) for the mixtures using the maximum likelihood method applied to the Redlich-Kister, Wilson, NRTL and UNIQUAC equations taking into account the vapour phase imperfection in terms of the second virial coefficient. (C) 2005 Elsevier Ltd. All rights reserved.