Adverse interactions of zinc dialkyldithiophosphate (ZDPs) with other required additives in an engine oil is one factor contributing to the high phosphorus content of some engine oils. Sine phosphorus poisons exhaust gas emission catalysts, ways to reduce the adverse interactions are being sought. The ZDP-amine interaction was investigated as a model of such an interaction. Above a critical concentration, amines can cancel the antiwear performance of ZDPs as measured in a 4-ball machine. At amine concentrations below the critical value, a synergistic antiwear effect was observed at higher loads. The tolerance of different ZDPs to the presence of amines was the same as the ranking of the ZDPs in terms of antiwear performance. A mechanism for the antiwear action of ZDPs involving the formation of an acid phosphate decomposition product has been proposed.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference in San Francisco, California, August 18–21, 1980.