Thermodynamic properties of mixtures of dipolar and non-polar linear molecules are obtained fromNpTmolecular dynamics (MD) simulations and are compared with previously reported results fromNVTMD simulations, and from a perturbation theory for polar non-spherical molecular fluids. The interactions between the particles of the mixtures are all of the same two-centre Lennard-Jones (2CLJ) type, of elongationL= 0·505 with an additional axial point dipole on the polar molecules. For these mixturesNpTMD simulations were performed for various dipole moments and compositions for a statepoint previously investigated byNVTMD simulations. We show that the approximation used to calculate excess Gibbs energies at constant pressure from theNVTensemble gives correct results even if it is more computationally demanding. Additionally, we present results for the excess enthalpy and the excess volume, which are in good agreement with perturbation theory.