A simple annular resonator is presented in which it is possible to measure the speed of sound with an accuracy of about 0.05%. Results are presented for gaseous trifluoromethane at nine temperatures in the range (294 to 318) K and for hexafluoroethane at six temperatures in the range (281 to 321) K. Measurements have been made in the pressure range (1 to 50) bar or 1 bar to 90% of the vapor pressure, which ever is higher. For trifluoromethane, the expected minimum near the critical point has been observed. Both sets of experimental measurements have been compared with the speed of sound predicted by classical equations of state.