The gaseous speed of sound, the ideal-gas heat capacity at constant pressure, and the second Virial coefficient were determined for pentafluoroethane (HFC-125). A total of 49-data points of speed of sound for gaseous HFC-125 were measured for temperatures from 273 to 313 K and pressures from 32 to 479 kPa with a cylindrical, variable-path acoustic interferometer. The ideal-gas heat capacity at constant pressure and the second acoustic Virial coefficient were determined over the temperature range from the speed of sound measurements and were correlated as functions of temperature. An analytical expression for the second Virial coefficient derived using the square-well intermolecular potential model was compared with the data.