The thermal conductivity of the slowly dissociating system 2NO2â 2NO + O2 has been measured from above 200 to 400°C by the hot wire technique. On comparison of the experimental data with the theories of heat transfer in reacting gases, it is found that the effect of chemical reaction upon the heat conductivity is apparent at 350°C and higher temperatures, while at 300°C and below it is negligibly small. However, the present theories of heat conductivity in reacting gas mixtures are found to be inadequate to account for the effects quantitatively.