An exhaust system for a lean-burn internal combustion engine comprises a catalyst for oxidising carbon monoxide and hydrocarbons comprising a flow-through substrate monolith comprising a first platinum group metal (PGM) zone containing at least one PGM supported on a surface area-increasing washcoat component, a second PGM zone containing at least one PGM supported on a surface area-increasing washcoat component, a third PGM zone containing at least one PGM supported on a surface area-increasing washcoat component wherein both the PGM loading in the first PGM zone and the PGM loading in the third PGM zone is greater than the PGM loading in the second PGM zone and wherein the first PGM zone comprises a washcoat loading that is less than a washcoat loading of the third PGM zone.