Densities, relative permittivities, and refractive indices were measured for the binary mixture gamma-butyrolactone + p-xylene from 288.15 K to 308.15 K. The above quantities are represented with polynomial smoothing curves. Excess volumes, as well as deviations of molar polarization, were calculated and found to be negative, with a minimum at mole fraction 0.5. The Kirkwood correlation factor was also calculated.