Speeds of sound have been measured at pressures up to 400 MPa in pure hexafluoropropene (common name hexafluoropropylene, HFP) and trifluoro-3-(trifluoromethyl)oxirane (common name hexafluoropropylene oxide, HFPO) along isotherms from T = (253.27 to 473.14) K and T = (253.15 to 403.15) K, respectively. The measurements were performed with a double-path pulse-echo apparatus at a frequency of 5 MHz. The ultrasonic cell was calibrated with degassed ultrapure water at T = 298.15 K and p = 1 MPa with reference to the International Association for Properties of Water and Steam equation of state (IAPWS-95). Due to the wide ranges of experimental temperatures and pressures investigated here, the speeds of sound were obtained for both components at conditions ranging from the subcritical liquid to the supercritical fluid. The standard uncertainties are estimated to be 0.015 K for temperature and 0.05 MPa for p <= 90 MPa and 0.0006p for p > 90 MPa for pressure. For the speed of sound c, the standard uncertainties are 0.8 m.s(-1) in HFP and 0.8 m.s(-1) for c <= 800 m.s(-1) and 0.001c for c > 800 m.s(-1) in HFPO.