AbstractThis paper reports work on the relationship of surface tension of ZDDP solutions to adsorption isotherms. The surface tension of the solutions varies with their composition, and information about adsorption, decomposition or other chemical changes of ZDDP in solution can be obtained from the correlation of solution concentration and surface tension. From this work, the authors conclude that adsorption of ZDDP on iron can occur at even low concentrations, increasing with temperature, and that decomposition, association and interaction between components may occur in solution at high temperatures.