Temperature variation in the combustion chamber of spark ignition engines is a vital factor in determining exhaust pollutant concentrations. Oxides of nitrogen are particularly affected. The temperature and concentration variations were investigated both theoretically and experimentally.A nonuniform model of the combustion process was developed. Calculations based on this model show that a temperature difference of the order of 600 K can be established across the cylinder. The validity of this model was substantiated by results of infrared spectroscopic measurements in the operating engine cylinder.The kinetic mechanism for formation of nitric oxide was used, along with the nonuniform combustion model, to investigate the formation of nitric oxide in the cylinder. Results of the kinetic calculation show that the temperature gradient across the cylinder has a profound effect on the nitric oxide formation. The calculations are shown to be in agreement with results of direct cylinder sampling for a variety of engine variables including fuel-air ratio, spark timing, and engine speed.