A series of bismuth carboxylates were tested with active and inactive sulfur compounds as extreme pressure (EP) lubricant additives. Wear and load measurements from four-ball tests are reported. Bismuth 2-ethylhexanoate (Bi 2EH), bismuth neodecanoate (Bi NDA) and bismuth naphthenate (Bi Nap) showed excellent EP activity with both active and inactive sulfur compounds (≥ 620 kg four-ball weld load). The mechanism of the EP lubricity was studied using thermogravimetric analysis (TGA), high-temperature X-ray diffraction (HTXRD), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The results of these studies support the formation of bismuth sulfide during thermolysis of the bismuth carboxylate-polysulfide mixtures. The structure of bismuth sulfide suggests that interlayer shear occurs and provides lubricity.