Abstract: The invention relates to a catalyzer installation for spark-ignition engines which are operated with air/fuel ratios of .lambda.1, particularly for boat engines, the catalyzer being subdivided into a reduction part located upstream in the exhaust gas line and an oxidation part located coaxially downstream after it. An intermediate space is located between the reduction and oxidation parts. Both catalyzer parts are surrounded by a preferably cylindrical, water-cooled casing and the casing has a downstream secondary air inlet to which a secondary air blower can be connected, the secondary air separating the very hot catalyzer from the double-walled, water-cooled casing and, in particular, flowing round the oxidation catalyzer part in counterflow for air preheating so that the air preheated in this manner is passed through the intermediate space into the oxidation part. The catalyzer is accommodated in a cartridge which is supported in wire compression rings in the catalyzer casing.