To reduce the number of particles in an exhaust gas (A), in particular in the exhaust gas (A) from an internal-combustion engine, sulfur dioxide SO₂ which is contained in the exhaust gas (A) is at least partially catalytically oxidized, in the presence of ammonia NH₃ in a catalytic converter system having a catalytic activity which increases in the direction of flow, to form sulfur trioxide SO₃. The simultaneous presence of sulfur trioxide SO₃ and ammonia NH₃ in the exhaust-gas stream which is brought about in this way results in an agglomeration of the small and very small particles and reduction in the number thereof.