Interactions between zinc dithiophosphate (ZDDP) antiwear additive and commercial calcium sulfonate detergent are studied using a plane/plane contact configuration friction machine lubricated under boundary conditions. Comparative tests have allowed us to envisage three types of interactions, respectively, due to, — first: chemical interaction between both additives in the oil phase leading to an effective ZDDP concentration decrease. — second: detergent effect due to the presence of the calcium sulfonate surfactant molecule, preventing basal materials from agglomerating during running. — third: specific role due to the overbasing agent, considered as a colloidal dispersion. When an overbased detergent is used, the two latter contributions are predominant. This study suggests that a new approach in boundary lubrication should be envisaged in relation to the evolution of the interface products which must be regarded as a highly concentrated colloidal system.