A vehicle comprising a diesel engine and an exhaust system comprising an oxidation catalyst for treating an exhaust gas from the diesel engine, wherein the oxidation catalyst comprises a substrate, which is a flow-through monolith and two layers only, wherein a first layer is disposed directly on the substrate and a second layer is disposed on the first layer, wherein: (a) the first layer comprises a platinum (Pt) component and a palladium (Pd) component as the only PGM components, and the second layer comprises a platinum (Pt) component and a palladium (Pd) component as the only PGM components, and wherein the ratio by mass of the platinum (Pt) component to the palladium (Pd) component in the first layer is different to the ratio by mass of the platinum (Pt) component to the palladium (Pd) component in the second layer; or (b) the first layer comprises a palladium (Pd) component and a platinum (Pt) component as the only PGM components, and wherein the second layer comprises a Pt component as the only PGM component; or (c) the first layer comprises a platinum (Pt) component as the only PGM component, and the second layer comprises a platinum (Pt) component and a palladium (Pd) component as the only PGM components, and a support material consisting essentially of alumina substitution doped or interstitially doped with silica at 1.5 to 10% by weight silica, wherein the first layer comprises the support material comprising the alumina doped with silica and the at least one platinum group metal component is supported on the support material; and an alkaline earth metal component, which is supported on the support material, wherein at least the first layer comprises the alkaline earth metal component; and wherein the platinum group metal component, the alkaline earth metal component and the support material are disposed on the substrate, wherein the total amount of the alkaline earth metal component is 0.35 to 17.66 gL -1 (10 to 500 gft -3). (a) the first layer comprises a platinum (Pt) component and a palladium (Pd) component as the only PGM components, and the second layer comprises a platinum (Pt) component and a palladium (Pd) component as the only PGM components, and wherein the ratio by mass of the platinum (Pt) component to the palladium (Pd) component in the first layer is different to the ratio by mass of the platinum (Pt) component to the palladium (Pd) component in the second layer; or (b) the first layer comprises a palladium (Pd) component and a platinum (Pt) component as the only PGM components, and wherein the second layer comprises a Pt component as the only PGM component; or (c) the first layer comprises a platinum (Pt) component as the only PGM component, and the second layer comprises a platinum (Pt) component and a palladium (Pd) component as the only PGM components, and a support material consisting essentially of alumina substitution doped or interstitially doped with silica at 1.5 to 10% by weight silica, wherein the first layer comprises the support material comprising the alumina doped with silica and the at least one platinum group metal component is supported on the support material; and an alkaline earth metal component, which is supported on the support material, wherein at least the first layer comprises the alkaline earth metal component; and wherein the platinum group metal component, the alkaline earth metal component and the support material are disposed on the substrate, wherein the total amount of the alkaline earth metal component is 0.35 to 17.66 gL -1 (10 to 500 gft -3). and a support material consisting essentially of alumina substitution doped or interstitially doped with silica at 1.5 to 10% by weight silica, wherein the first layer comprises the support material comprising the alumina doped with silica and the at least one platinum group metal component is supported on the support material; and an alkaline earth metal component, which is supported on the support material, wherein at least the first layer comprises the alkaline earth metal component; and wherein the platinum group metal component, the alkaline earth metal component and the support material are disposed on the substrate, wherein the total amount of the alkaline earth metal component is 0.35 to 17.66 gL -1 (10 to 500 gft -3).