Several reviews have been published in the past few years summarizing various methods of calculating mixing thermal conductivities, and large number of theoretical papers have also appeared discussing the problems, approximations, and limitations of the various methods. The theory for calculating the conductivity for rare-gas mixture has been worked out in detail. The more difficult problem, however, is to modify monatomic mixture correlation to apply to polyatomic molecules. Many techniques have been proposed; all are essentially empirical, and most reduce to some form of the Wassiljewa equation.