A process for operating a marine engine (1) in combination with a catalytic partial oxidation reformer (2) and, optionally, an exhaust gas aftertreater (3) , wherein: (a) a mixture of a first fuel and air, is introduced in the combustion chamber of the engine (1); (b) exhaust gas is discharged from the engine and optionally partly recirculated to the combustion chamber of the engine (1); (c) a second fuel and oxygen and/or steam are supplied to the catalytic partial oxidation reformer (2) to produce synthesis gas, wherein the second fuel comprises Fischer-Tropsch derived fuel; (d) at least part of the synthesis gas is supplied to: (i) the exhaust gas aftertreater (3); (ii) the combustion chamber of the engine (1); or to both.