A radiochemical analysis has been made of the EP film formed on the surfaces of the tappets of an engine lubricated with an oil containing triphenyl phosphate, labeled with phosphorus –32, as the EP additive. The results do not support the widely held view that phosphate esters owe their effectiveness as EP additives to the formation of phosphide eutectics of low melting point on metal surfaces. The results indicate the presence of metal phosphates and metal organophosphates formed from acid-phosphate intermediates derived from the ester. These findings, together with the results of subsequent rig tests, suggest that the effectiveness of neutral organic phosphates as EP additives depends on the ease with which they hydrolyze on the metal surface to form acid phosphates; the more readily the ester hydrolyzes the better its performance.