At present the primary difficulty with employing carbon for the purposes of NOx removal is the lack of selectivity. Carbon generally reacts as readily with oxygen as with NO. Thus, carbons in lean-burn exhausts (O2/NO>100) generally `burn' rather than selectively removing NO. Here we report on microcalorimetric studies which show that high temperature (950 °C) hydrogen-treated carbons will adsorb NO at room temperature, but will not adsorb significant quantities of oxygen. In contrast, the same activated carbon treated at high temperature in nitrogen will strongly adsorb both species. The selective nature of the hydrogen-treated material and the less-selective nature of the nitrogen-treated material is fully consistent with an earlier model of carbon surface chemistry which highlights the contrast in the character and concentration of active surface species created by each of these treatments.