A method is described for obtaining the dielectric constant and conductivity of disulphur decafluoride in both liquid and gaseous forms. The freezing point of the liquid and the molecular weight of the vapor are established as criteria of purity. All the measurements lead to the conclusion that disulphur decafluoride is non-polar. Its molar polarizations in the two forms are compared with those of other members of the sulphur fluoride and carbon fluoride homologous series as well as with hexane, benzene, and toluene.