An exhaust system for a spark-ignited internal combustion engine comprises a three-way catalyst composition including an oxygen storage component, which TWC composition is coated on a flow-through monolith substrate, which substrate comprising a plurality of channels, each channel having a length extending from an inlet end to an outlet end and a single lambda sensor, wherein the substrate comprises a portion of the plurality of channels wherein the TWC composition in at least a part of the length of channels extending from the inlet end has a reduced oxygen storage activity, or no oxygen storage activity, relative to the TWC composition in a remainder of the substrate, the arrangement being such that the single lambda sensor is contacted substantially only with exhaust gas that has first contacted the TWC composition having a reduced oxygen storage activity or no oxygen storage activity.