The combustion of high-level-sulfur (2-5%) fuels generates large amounts of sulfuric acid in slow-speed crosshead marine diesel engines. In this study, a video-microscope imaging technique is used to develop an experimental protocol for ranking the ability of marine cylinder lubricants to neutralize sulfuric acid within microcapillaries. The technique was applied to several oils with known rates of acid-neutralization ability to predict these rates. The optical observations allow one to propose a plausible mechanism by which the overbased detergent additive system performs the task of acid neutralization. According to this mechanism the overbased sulfonate reverse micelles are the sire for the neutralization reaction, which occurs at the oil-acid interface. The results confirm that the detergent surfactants are chemically bound to the micellar core. in the case of overbased calcium-carbonate detergents reacting with sulfuric acid, the formation of solid crystals was observed which may form a protective antiwear film on engine metal surfaces.