A method of preparing a catalyst composition for producing a stable ratio of NO2 to NO in an exhaust system of a compression ignition engine is described. The method comprises: (i) preparing a first composition comprising a platinum (Pt) compound disposed or supported on a support material; (ii) preparing a second composition by reducing the platinum (Pt) compound to platinum (Pt) with a reducing agent; and (iii) heating the second composition to at least 650 DEG C.