The paper presents accurate representations for the thermal conductivity of the diatomic gases nitrogen and carbon monoxide in the limit of zero density. These gases were studied because they have nearly the same molecular mass and viscosities. In contrast, the new analysis confirms that the thermal conductivities of the two gases differ remarkably, especially at low temperatures. The theoretically‐based correlations provided are valid for the temperature range 220–2100 K and have associated uncertainties of ±1% between 300 and about 500 K, rising to ±2.5% at the low‐ and high‐temperature extremes. A comparison with some empirical and semiempirical correlations is given.