ABSTRACTThe water‐shedding property and the interfacial tension of marine engine oils with various additives have been assessed and analysed in this paper. It is concluded that the weak water‐shedding property was mainly due to the molecular structure and the interfacial property of sulphonate detergent, which can adsorb on the oil–water interface intensely to form a strong interfacial film and prevent coalescence of droplets. The impact of a demulsifier to the water‐shedding property and the interfacial tension indicated that demulsifier improved the water‐shedding property of marine engine oil in the manner of being much easy to adsorb onto the interface but formatting a flimsy interfacial film. Copyright © 2011 John Wiley & Sons, Ltd.