Novel quaternary ammonium thiomolybdates are prepared by contacting an aqueous solution of a thiomolybdate salt with a solution of a water-insoluble quaternary ammonium salt in a water-immiscible organic solvent. For example, mixing a solution of dicocodimethylammonium chloride in toluene with an aqueous solution of potassium thiomolybdate results in a toluene solution containing dicocodimethylammonium thiomolybdate. These quaternary ammonium thiomolybdates find use as additives in motor oils and greases to enhance their antifriction characteristics.