Exhaust manifold reactor systems using air injection have been developed to control exhaust emission levels of a variety of vehicles to less than 50 ppm hydrocarbons and 0.5% carbon monoxide. These systems have been shown to be compatible with other control systems used to reduce evaporative emissions and oxides of nitrogen. Manifold reactors were constructed which were sufficiently durable to control emissions, effectively for 100,000 miles when run in accordance with the mileage accumulation schedule suggested in the federal emission control device certification procedure.Problem areas hampering immediate application of reactor systems are described. Manifold reactor units are large and will not fit in the engine compartments of many vehicles; furthermore, some current engine configurations are not suited to the installation of reactors. Current reactor construction techniques are costly and materials of proven durability are relatively expensive. In addition, at the present stage of development, some carburetor enrichment is required which results in lower fuel economy. While preliminary work has indicated that these problems are probably amenable to solution, much additional development will be required.