The experimental techniques for the measurement of thermal conductivity of gases at very high temperatures were examined. Using a wall stabilised are the thermal conductivity of argon and nitrogen was measured in the temperature range 8000-16,000°K and compared with published experimental and theoretical data. Reasons for the very significant discrepancies between theory and experiment at the higher temperature end of the range examined were sought and the importance of thermal radiation demonstrated. The problems involved in including the thermal energy contribution to the arc energy balance were examined together with the measurement and calculation of the radiation density. A method for the direct measurement of viscosity using the wall stabilised arc was examined and a new method for the measurement of velocity in high temperature gases was developed. Following the experimental measurement of gas properties, the problems involved in their theoretical prediction were examined. A new method of calculation of thermal conductivity and viscosity of gases with non-frozen composition was developed which, whilst maintaining a high level of accuracy in the temperature range up to about 18,000°K in argon and nitrogen at atmospheric pressure offered a very considerable simplification and reduction in labour over other published methods. The final section of this study was concerned with the evaluation of electron-proton and electron-ion collision data in the arc. The absorption of electromagnetic radiation produced by low power He-Ne and GaAs lasers was used to make the measurements; the importance of the electron-ion collisions being clearly demonstrated in this work.