In view of the large (and further increasing) range of fuels applied in marine diesel engines, there is a clear need for obtaining a better understanding of the effect of those fuels on the key in-cylinder processes governing the combustion characteristics of these engines. For this purpose, a constant volume chamber representative of the combustion system of large marine diesel engines has been complemented with a device allowing the investigation of small fuel quantities and the resulting setup has been used for studying the combustion behaviour of typical marine diesel fuels at conditions relevant for large marine two-stroke diesel engines. Specifically, two clearly distinct heavy fuel oils have been compared to a light fuel oil.Two optical measurement techniques were used to complement the findings made on the basis of rate of heat release analysis. A combination of the shadow imaging technique with the OH*-chemiluminescence method was applied to investigate spray tip penetration, ignition delay, ignition location, flame lift off as well as flame development of the heavy fuels in comparison with a light fuel oil.The ignition delay was found to be more than doubled with the heavy fuels, whereas the distance of the ignition spot from the injector was only marginally affected. During the very early combustion, light fuel oil burned quicker than the heavy fuels, but once the conditions were right to allow fast evaporation of the heavy fuels, the flame expansion was at comparable rates for all fuels. The shape and position of the flame area in the later stage on the other hand also exhibits a distinct influence of fuel quality as the area covered by the flame is clearly larger for the heavy fuel oil cases. At the same time, the flame lift-off for those fuels is also increased compared to the reference fuel. However, the evaluation of the overall combustion behaviour on the basis of the heat release rate analysis data shows that this difference in initial behaviour disappears during the main combustion phase. In fact, the overall combustion quality was virtually the same for all cases.