An arrangement for treating exhaust gases of an internal combustion piston engine in a marine vessel, includes an exhaust gas channel coupled at its first end to the engine and having its second end opening to the environment, and a scrubber unit arranged between the first end and the second end of the exhaust gas channel. The arrangement has an exhaust gas channel cleaning system arranged to apply cleaning liquid on inner surface of the exhaust gas channel to a portion of the exhaust gas channel between the scrubber unit and the second end of the exhaust gas channel.