The effect of water concentration on the foaming tendency of lubricating oil containing the detergent additives Ca-phenate and amide was investigated from a surface chemical standpoint. Foam volume, dynamic surface tension and surface viscosity were measured at various water concentrations. It was proved that the increase of surface viscosity with water content was the main factor affecting foaming in the case of Ca-phenate but, in contrast, the Marangoni effect played an essential role in the case of amide.