We report comprehensive thermodynamic property measurements of 1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)-3-pentanone. The (p, rho, T) behavior was measured from T = (225 to 470) K with pressures up to 36 MPa with a two-sinker densimeter. These measurements include compressed-liquid states and states in the extended critical region. The vapor-phase speed of sound was measured from T = (325 to 500) K with pressures up to 1.7 MPa with a spherical acoustic resonator. The vapor pressure was measured in the spherical resonator from T = (325 to 440) K with a static technique. The density and speed of sound of the liquid was measured from T = (278 to 308) K at atmospheric pressure (p = 83 kPa) in a benchtop instrument employing a vibrating-U-tube densimeter and a time-of-flight speed-of-sound technique. These data, together with selected data from the fluid manufacturer, have been used to develop an equation of state explicit in the Helmholtz energy covering the fluid region. The equation of state represents the present experimental vapor-pressure data with an RMS deviation of 0.066 %, the (p, rho, T) data to 0.067 %, and the speed-of-sound data to 0.029 %. This fluid is of interest as the working fluid in Rankine-cycle power applications and as a fire extinguishing agent; it is also known by the trade names Novec-649 and Novec-1230.