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Capillary electrophoresis has been successfully applied to the separa-Ž . tion of four different commercial zinc dialkyldithiophosphate ZDDP mixtures and to the analysis of the ZDDP in fresh and used lubricating oils. Nonaqueous systems composed of methanol, tetrahydrofuran, and acetonitrile with tetraethylammonium, tetramethylammonium, or ammonium hydroxide and acetic acid as buffer additives were employed to separate the metal complexes according to the different hydrocarbon chain lengths and structures of the alkyl substituents.