The fundamental principle underlying the selective catalytic reduction (SCR) of NOx to N-2 is the promotion of reactions of reductant with NOx over competing, and thermodynamically preferred, reactions with a large excess of O-2. A similar competition between NOx and O-2 exists in the noncatalytic, thermal reduction of NOx with NH3. In this work, density functional theory calculations are used to elucidate the origins of the remarkable selectivity in thermal deNO(x). Thermal deNO(x) is initiated by the conversion of NH3 into the active reductant, NH2 radical. NH2 radical reacts with NO at rates typical of gas-phase radical reactions to produce a relatively strongly bound H2NNO adduct that readily rearranges and decomposes to N-2 and H2O. In contrast, NH2 radical reacts exceedingly slowly with O-2: the H2N-OO adduct is weakly bound and more readily falls apart than reacts to products. The pronounced discrimination of NH2 against reaction with O-2 is unusual behavior for a radical but can be understood through comparison of the electronic structures of the H2NNO and H2NOO radical adducts. These two key elements of thermal deNO(x)-reductant activation and kinetic inhibition of reactions with O-2-are similarly essential to successful catalytic lean NOx reduction, and are important to consider in evaluating and modeling NOx SCR.