Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NOx and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N-2 and O-2) conditions, the NO decomposition (NO <----> (1/2)N-2 + (1/2)O-2) and NO oxidation (NO + (1/2)O-2 <----> NO2) equilibria impose lower bounds on the NOx concentrations achievable by thermodynamic equilibration or NOx decomposition, and these equilibrium NOx concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NOx levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier 11 NOx emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO, values can fall below NO, emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NOx removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NOx standards will require selective NOx catalysts that vigorously promote NOx reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NOx catalyst system to reduce NOx concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity.