Many methods of altering the composition of internal combustion engine exhaust gases have been considered for reducing automotive air pollution. By providing a low-energy reaction path, catalysis may be the most effective method of reducing automotive air pollution. Since chemical equilibrium represents the ultimate limit of catalytic treatment of engine exhaust, equilibrium compositions were determined for nine air-fuel mixtures, with overall air-fuel ratios (A/Fs) ranging 12: 1-17: 1 (lb air/lb fuel), for temperatures 300-1500 F. A NASA computation procedure based on free-energy minimization was used to determine the equilibrium concentrations.These calculations indicated that true equilibration of exhaust gases, in the temperature range considered, would nearly eliminate the major exhaust pollutants. No hydrocarbons (HCs), except nonreactive CH4, appeared in significant concentrations. Even at the leanest A/F considered, equilibrium NO concentrations were less than 30 ppm. Thus, the two major reactants in the formation of photochemical smog, HC and NOx, are essentially eliminated when the exhaust gases are equilibrated. Only CO, of the major automotive air pollutants, is present in significant concentrations at equilibrium, and then only at rich A/F. True equilibria CO concentrations are less than 0.25% for all A/F greater than 14.6.The results also suggest that the formation of NH3, by NO-reduction catalysis is caused by kinetic limitations which prevent the catalytic formation of HCs. The formation of NH3 provides an alternate free-energy “minimum” for the exhaust gas system because of these kinetic limitations.