A radioactive tracer, phosphorus-32 synthesized into zinc dialkyl dithiophosphate molecules, was used to study the EP films formed by this motor oil additive on cast iron cams and tappets. The tightly bound films, which contain phosphorus and zinc, formed rapidly and then approached steady-state values in static and dynamic tests. The amount of film increased with oil temperature (200 to 300F) and with available surface area. Dynamic test conditions greatly increased the amount of film formed, indicating the marked effect of surface temperature, pressure or wear. The film is maintained on parts run in additive oil; moreover, the film was not easily worn off in non-additive oil. The chemical and physical properties of the films appeared to vary with the conditions of film formation. Radioactive counting was used to determine the film on tappets and the thermal stability of the compounded oil. Autoradiographs showed the concentration and distribution of the film on cams and tappets.