The environmental impact of exhaust gas pollution from ships is receiving increasing attention. Due to the characteristics of the combustion process and the use of heavy fuels, the marine diesel engine is one of the largest pollutants of the sea and air because it emits significant amounts of harmful substances such as: nitrogen oxides (NOx), sulfur oxides (SOx) carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC) and soot particles. For this reason, the International Maritime Organization (IMO) has laid down a number of regulations that limit, control and prevent the release of harmful substances in large quantities. Knowledge of the process of formation of harmful substances, in what quantities they are produced and their harmful effects are an important factor for the improvement of marine engines by manufacturers, as well as for further tightening of regulations regarding the limitation of emissions of harmful substances. This article describes the combustion process in two-stroke marine engines, the process of generating exhaust gases as a combustion product, describes the techniques and methods used on ships today to reduce harmful emissions, and gives an example of measuring and analyzing the composition of exhaust gases.