Values of gaseous transport properties at elevated temperatures may be calculated from appropriate kinetic theory relations (or from statistical mechanics if equilibrium properties are of interest) provided that the correct intermolecular potential functions are known. It is pointed out that the potentials for this purpose may not be extrapolations of functions which are valid only for relatively large distances of separation, and therefore suitable only for calculation of low temperature properties, but must be ones which are valid at the smaller distances of separation of importance at elevated temperatures. The experimental procedure is described for determining such potentials from elastic scattering of neutral beam particles having kinetic energies of the order of 1000 eV. It is shown that meaningful potential energy information cannot be deduced from such experiments unless the shape and intensity distribution of the beam, as well as the geometry of the beam-detector system are taken into account. Results are tabulated for the potentials of a number of atom-atom and atom-molecule systems and a procedure for obtaining molecule-molecule interactions from such results is indicated. The procedure is outlined for using appropriate interaction potentials for calculating transport properties over a wide range of temperature and the results are illustrated in terms of the viscosity of xenon between 1000° and 10,000°K. It is shown that the method may be applied to the calculation of properties of mixtures which require potentials between unlike particles. The criterion is given for determining the range of temperature over which all such calculations are valid, namely, that it is the magnitude of the fraction of the kinetic energy of the beam particles which is converted into potential energy at the distance of closest approach during a collision and not the kinetic energy itself, which determines the region of validity. Possible experimental procedures are suggested which might be used to obtain intermolecular potentials for binary systems containing ions, dissociative atomic species, metastable and excited species.