Isobaric vapor-liquid equilibrium data were obtained for the system p-xylene + decane at 97.3 kPa using a vapor recirculating type (modified Othmer's) equilibrium still. The activity coefficients were obtained by taking into consideration the vapor-phase nonideality. A comparison of values of activity coefficients obtained by experimental data was made with UNIFAC. The experimental data for the system were tested for thermodynamic consistency and correlated by various equations. Also the excess molar volumes, viscosities, and speeds of sound for the binary mixture at T = (303.15, 308.15, and 313.15) K had been measured over the whole compositional range. The excess thermodynamic properties such as deviations in viscosity (Delta eta) and excess isentropic compressibility (K-S(E)) were calculated.