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This paper investigates active control of an aftertreatment system for a conventional spark ignition engine equipped with one or two three-way catalysts and two oxygen sensors. The control objective is to maximize the simultaneous conversion efficiencies of oxides of nitrogen and unburned hydrocarbons. Linear exhaust gas oxygen (EGO) sensors are used to measure air-fuel (A/F) ratio upstream and downstream of each catalyst. A series controller configuration is adopted. The upstream controller provides relatively rapid response to disturbances on the basis of measured feedgas A/F ratio, while the downstream controller uses the feedgas and post-catalyst A/F ratio measurements to compensate for the bias corrupting the feedgas A/F ratio measurement. The performance and robustness of the proposed control system in the face of noise and model uncertainty are first evaluated through extensive simulations. The control strategy is then experimentally verified in a dynamometer test cell and its performance compared with an existing proprietary controller that is based on the more common switching-type A/F ratio sensors.