Internal pressure ( i), free volume (Vf), enthalpy (H) and entropy (Ts) have been derived from the measured values of densities () and speeds of sound (u) for binary liquid mixtures of diethylcarbonate with o-xylene, m-xylene and p-xylene at T = (298.15 to 323.15) K over the entire composition range and at atmospheric pressure. By using these values, excess internal pressure ( iE), excess free volume (VfE), excess enthalpy (HE), excess free energy (GE) and excess entropy (TsE) have been calculated. The calculated excess values have been fitted to the Redlich-Kister polynomial equations and the results are analysed in terms of molecular interactions due to physical, chemical and structural effects between the unlike molecules.