A hot‐wire apparatus of the ``potential leads'' type has been used to determine the thermal conductivities of nitrogen, oxygen, nitric oxide and nitrous oxide, and the binary mixtures: N2O&sngbnd;N2, N2O&sngbnd;O2, N2O&sngbnd;NO and NO&sngbnd;N2, over the temperature range 30° to 200°C. Corrections were made for the various possible sources of error, and the final results are accurate to within 2%. The results for single gases are discussed in terms of the original Eucken formula, the diffusion‐mechanism theory and the Mason‐Monchick theory. The mixtures are treated in terms of the general Hirschfelder equation, and also by use of the Wassiljewa‐type relation. In the latter case, the experimental Wassiljewa coefficients are compared with those calculated by the Mason‐Saxena and Lindsay‐Bromley approximate relations.