An oxidation catalyst for treating hydrocarbons in an exhaust gas produced by an internal combustion engine, wherein the oxidation catalyst comprises a first region disposed on a flow-through monolith substrate or a filtering monolith substrate, wherein the first region comprises ruthenium (Ru) supported on a support material comprising a refractory oxide, wherein the refractory oxide comprises alumina or ceria-zirconia; wherein the oxidation catalyst further comprises a second region, wherein the second region comprises a noble metal supported on a second support material, wherein the noble metal is selected from the group consisting of platinum (Pt), palladium (Pd), a combination or a mixture of platinum (Pt) and palladium (Pd), and a combination or a mixture of palladium (Pd) and gold (Au); characterized in that: (i) the first region is a first zone disposed at or near an outlet end of the substrate and the second region is a second zone disposed at or near an inlet end of the substrate; (ii) the second region is a second layer and the first region is a first zone, and wherein the first zone is disposed on the second layer at or near an outlet end of the substrate; (iii) the second region is a second layer and the first region is a first layer, and wherein the second layer is disposed on the first layer; (iv) the second region is a second zone disposed at or near an outlet end of the substrate and the first region is a first zone disposed at or near an inlet end of the substrate; (v) the first region is a first layer and the second region is a second zone, and wherein the second zone is disposed on the first layer at or near an outlet end of the substrate; or (vi) the first region is a first layer and the second region is a second layer, and wherein the first layer is disposed on the second layer. wherein the oxidation catalyst further comprises a second region, wherein the second region comprises a noble metal supported on a second support material, wherein the noble metal is selected from the group consisting of platinum (Pt), palladium (Pd), a combination or a mixture of platinum (Pt) and palladium (Pd), and a combination or a mixture of palladium (Pd) and gold (Au); characterized in that: (i) the first region is a first zone disposed at or near an outlet end of the substrate and the second region is a second zone disposed at or near an inlet end of the substrate; (ii) the second region is a second layer and the first region is a first zone, and wherein the first zone is disposed on the second layer at or near an outlet end of the substrate; (iii) the second region is a second layer and the first region is a first layer, and wherein the second layer is disposed on the first layer; (iv) the second region is a second zone disposed at or near an outlet end of the substrate and the first region is a first zone disposed at or near an inlet end of the substrate; (v) the first region is a first layer and the second region is a second zone, and wherein the second zone is disposed on the first layer at or near an outlet end of the substrate; or (vi) the first region is a first layer and the second region is a second layer, and wherein the first layer is disposed on the second layer. (i) the first region is a first zone disposed at or near an outlet end of the substrate and the second region is a second zone disposed at or near an inlet end of the substrate; (ii) the second region is a second layer and the first region is a first zone, and wherein the first zone is disposed on the second layer at or near an outlet end of the substrate; (iii) the second region is a second layer and the first region is a first layer, and wherein the second layer is disposed on the first layer; (iv) the second region is a second zone disposed at or near an outlet end of the substrate and the first region is a first zone disposed at or near an inlet end of the substrate; (v) the first region is a first layer and the second region is a second zone, and wherein the second zone is disposed on the first layer at or near an outlet end of the substrate; or (vi) the first region is a first layer and the second region is a second layer, and wherein the first layer is disposed on the second layer.