Pollutants from the exhaust of a lean running internal combustion engine are reduced by the engine having an exhaust catalyst capable of oxidising hydrocarbons and carbon monoxide and reducing NO x to nitrogen and being fitted with engine management means arranged to provide transient additional enrichment during periods of acceleration. A novel exhaust catalyst for this purpose is carried by a washcoat and is capable of oxidising hydrocarbons and carbon monoxide and reducing NO x to nitrogen and contains in addition 0.1 - 20% by weight of iron based on the total weight of catalyst and washcoat.