Oxidation catalyst for treating the exhaust gas of a compression ignition engine

A method of treating carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas from a compression ignition engine, which method comprises contacting the exhaust gas with an oxidation catalyst, wherein the oxidation catalyst comprises one or more layers or zones comprising:

• a platinum group metal (PGM) component selected from the group consisting of (i) a platinum (Pt) component and (ii) a combination of a platinum (Pt) component and a palladium component, wherein the ratio by mass of the combination of the platinum (Pt) component to the palladium (Pd) component is >2:3;
• an alkaline earth metal component;
• a support material comprising a modified alumina incorporating a heteroatom component;
• and
• a substrate, which is a flow-through monolith;
• and
• wherein the platinum group metal (PGM) component, the alkaline earth metal component and the support material are disposed on the substrate, wherein the alkaline earth metal component is disposed or supported on the support material and wherein the total amount of the alkaline earth metal component is 10 to 500 gft−3.