Adsorption of zinc di-n-octyldithiophosphate synthesized with radioactive 65Zn has been studied on iron, steel, silica, and carbon. Adsorption on iron powder in an inert atmosphere is described by Elovich kinetics and Langmuir type isotherms. Adsorption equilibrium was not complete even after ten days. Results indicate activated chemisorption with an apparent activation energy of 15 kJ/mol (3.6 kcal/mol) on relatively uniform adsorption sites. Adsorbent composition, oxygen concentration, and solution contaminants, such as methanol, strongly influence adsorption capacity. Monolayer coverage, derived from the Langmuir isotherm, is in reasonable agreement with differently derived literature values. Pronounced variations observed on different adsorbents are thought to be caused by catalytic decomposition or oxidation of ZDDP. Because of this sensitivity, it appears that laboratory studies with simplified systems cannot provide much information on the actual structure of an antiwear layer formed in practical multicomponent lubricants.