In general, metal-based detergents are used in engine oil to prevent the adhesion and accumulation of deposits and sludge. Because metal-based detergents are additives composed of fatty acid metal salts and carbonates, such as calcium, they are known to be easily affected by water. In particular, over-based metal-based detergents containing a large number of carbonates, which are excellent in terms of detergency, have been used often in recent years. Following the increasing use of biofuels and the diffusion of hybrid vehicles, the risk of water being mixed into engine oil has increased. For example, in hybrid vehicles, it is assumed that the engine oil temperature decreases and it becomes difficult for water to volatilize. Also, the E100 fuel contains several percent of water. Therefore, it has become important to understand the degradation characteristics of engine oil when water is immixed, which is the aim of this study.1) In this study, we operated an engine while intentionally introducing water into its combustion chamber. Based on the evaluated degradation characteristics, we examined a laboratory test method and established the test conditions. Using the examined laboratory test method, we evaluated the degradation characteristics of engine oil when water is immixed. Consequently, we found that the principal factor that decreased the total base number when water was immixed was the presence of carbonates in metal-based detergents. We also revealed that the type of soap base also affects the amount of decrease in the total base number. In the future, engine oils containing detergents that have excellent water resistance are expected to be developed using the laboratory test method established in this study.