An exhaust system (10) for a dual fuel engine, which engine is adapted to operate in a first mode in which the engine is fuelled entirely by a liquid fuel, and a second mode in which the engine is fuelled predominately by a gaseous fuel, the system comprising a first oxidation catalyst (22), a first temperature sensor (30) adapted to measure a core temperature of the first oxidation catalyst, and a controller (32) for controlling whether the engine operates in the first mode or the second mode, wherein the first temperature sensor is operatively connectable to the controller such that the controller prevents the engine from running in the second mode if the core temperature of the first oxidation catalyst is below a predetermined temperature.