Method for the preparation of a wall flow particulate filter catalysed at its inlet side with a first catalyst having activity in the removal of residual hydrocarbons and carbon monoxide and catalysing at rich burn engine operation conditions the reaction of nitrogen oxides with hydrogen and/or carbon monoxide to ammonia and catalysed at its outlet side with a second catalyst having activity in the selective reduction of NOx by reaction with ammonia being formed in the inlet side. The method involves the provision of a first catalyst having a particle size smaller than the filter wall mean pore size, and a second catalyst having a particle size larger than the filter wall mean pore size, and mixing the first and second catalyst into one suspension, which is used for wash-coating from the outlet end. The first catalyst thereby diffuses into the partition wall.