When catalytic converters are used for cleansing of exhaust gas from two - stroke marine engines, new issues must be brought up in combination with prior technology. Therefore, a study was made of V6, 2600cc engine having a large volume of hydrocarbon emissions with respect to three issues: (1) To what degree seawater effects catalytic converter performance and possible countermeasures (2) Effects attained on cleansing level and catalyst temperature (3) Finding abatement levels for catalyst deterioration and exhaust emission output in the marine mode. It was found that physical adsorption was a significant factor in catalytic degradation resulting from direct contact with seawater. The cleansing levels obtained when a marine engine is equipped with a catalyst converter were found by clarifying the extent of effects of catalyst volume, performance and temperature. The reduction obtained in exhaust emission allowing for a deterioration factor, is shown in a catalytic converter heated to the maximum temperature of 960°C.