The invention relates to a method of determining activity and aging behavior of a catalyst. For this purpose, two independent substreams of combustion offgases are produced and mixed before coming into contact with the catalyst. The first substream of combustion offgases is preferably produced by burning a motor fuel and forms the major part of the combustion offgases. The second substream of combustion offgases is produced by means of a gas burner whose air ratio can be adjusted within a wide range. Mixing of this second substream of combustion offgases into the first substream enables the composition of the total stream to be set in a defined manner.