Automotive thermal reactors have obtained high conversion efficiencies on engines with very rich carburetion, but fuel economy and reactor durability have suffered. Improved mixing of exhaust gas and secondary air in the engine exhaust port was examined as a means of improving reactor efficiency at less rich engine air-fuel ratios. Three air-injection systems which span a broad range of mixing capabilities were examined. Mixing characteristics were deduced from anemometry measurements of instantaneous secondary airflow, and emission performance of each system was generalized by a test program employing four steady-state conditions.High-pressure, timed air injection provides the best mixing and the best reactor performance. Sparger (radial discharge) air injection tubes provide fair mixing and better performance than conventional open-ended air injection tubes, which exhibit poor mixing characteristics. Performance with sparger tubes is significantly poorer than with timed injection, but sparger tubes are more practical in terms of cost, complexity, and durability.