Effects of gas mean residence time and residence time distribution on the emissions performance of automotive thermal reactors were evaluated using a research reactor which provided independent control of volume, internal configuration, and heat loss. Steady-state tests of this reactor on a lean-mixture, multicylinder engine revealed that mean residence time significantly influences the performance of well-insulated reactors. However, high heat loss diminishes the residence time effect in uninsulated reactors. Adjusting reactor configuration to improve residence time distribution increases performance irrespective of heat loss. Overall, insulated “2-pass” reactors are nearly optimum.