Results of warm-up and fuel economy studies on a dual catalyst system using GEM reinforced Ni-Cu NOx reduction catalysts and PTX®-II B oxidation catalysts are reported. Rapid warm-up of this system is required to control emissions to the target level of 3.4 g/mi. CO, 0.41 g/mi. HC, and 0.4 g/mi. NOx. This rapid warm-up can be obtained only by oxidizing the CO, HC, and H2 in the exhaust during the first 30-60 seconds after start-up. Methods of inducing oxidation are described.Controlling NOx to 0.4 g/mi. requires GEM catalyst temperatures of ∼1300°F., 2-300°F. hotter than exhaust temperature during warmed-up operation. The additional temperature can be generated by enriching carburetion and oxidizing the CO, H2, and HC formed, or by spark retard. Studies conducted on a 1973-350 CID Chevrolet indicate a ∼4%/100°F. fuel penalty for increasing temperature by richer carburetion and air bleed at constant spark timing, and a ∼8%/100°F. penalty for spark retard at constant carburetion. Reducing the fuel economy penalty associated with meeting stringent NOx emission standards with dual catalyst systems, will require the development of durable NOx reduction catalysts which show high activity at normal exhaust temperature.This is the fourth in a series of SAE papers describing Esso Research and Enineering Company's studies of the use of nickel-copper alloy catalysts for the control of automotive emissions of oxides of nitrogen (NOx). Previous papers have presented data identifying nickel-copper alloy as an active NOx reduction catalyst *, discussed the application of this catalyst in dual catalyst systems , and reported progress made in optimizing dual catalyst system performance around the characteristics of this catalyst . For the past three years this work has been carried out in cooperation with Gould Inc., who have developed advanced versions of this catalyst, referred to as GEM.The purpose of this paper is to report the latest results from our studies of factors affecting the level of emission control and the fuel economy of vehicles equipped with dual catalyst systems incorporating GEM NOx reduction catalysts. We will discuss catalyst system warm-up, its effect on emissions; and maintenance of catalyst temperature and its effect on emissions and fuel economy.