As part of recent IIEC-2 activities, we have studied kinetics of ammonia formation to delineate engine conditions for best net NOx efficiency and to expand the choice of emission control systems. These studies have been carried out in the laboratory with exhaust gases generated by a pulse flame generator and rhodium containing catalysts. Catalyst selectivity, NH3/N2 was found to vary linearly with the CO/O2 ratio in the exhaust gas and with the reciprocal of the space velocity. The effects of temperature and input NOx levels are more complex: Selectivity shows a maximum in the range of 750-1300°F and increases gradually as the NOx level is decreased from 800 - 100 ppm. Based on these results a mechanism is proposed for ammonia formation and a triple-catalyst system is suggested for increasing net NOx efficiency.