We report the thermodynamic properties of 1,1,1,2,2,3,3,4-octafluorobutane, which has been designated as HFC-338 mccq by the refrigeration industry. The measurements span the temperature and pressure ranges encountered in thermal machinery. The properties measured include the vapor pressure, the density of the compressed liquid, the refractive index of both the saturated liquid and the saturated vapor, the critical temperature, the capillary rise, and the speed of sound in the vapor phase. These data were used to determine the ideal gas heat capacity, the saturated liquid and vapor densities, the surface tension, the virial equation of state for the vapor phase, and estimates of the critical pressure and density. A Carnahan-Starling-DeSantis equation of state was deduced from the measured fluid properties.