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We review mantle redox models and present new data for xenolithic and megacrystic intrinsic oxygen fugacity (IOF) studies from varied geologic settings. The roles of fluid inclusions, carbon, autoreduction, auto-oxidation, Ti 3+ , crystal defects, disproportionation, metasomatism, and gravity are examined with regard to their possible influences on redox data. In several IOF studies, the geothermometric determination for a multimineral sample yields very close temperature concordance with totally different geothermometric techniques; these specific studies mandate some confidence in the IOF f O 2 values obtained. Also of high confidence are the IOF data that come from nearly flawless, gem-quality megacrysts (GQ) of various silicates. Both types of these high-confidence IOF data indicate that the redox state of the mantle is nearer the wüstite-iron (WI) buffer than the quartz-fayalite-magnetite (QFM) buffer. The f O 2 data of the ilmenite-containing xenolithic assemblages that have either been calculated (Eggler, 1983), IOF-measured (Arculus and others, 1984), or gas mixture-equilibrated (McMahon, 1984) have all shown reasonable overlap in f O 2 -T values, and all indicate that these samples come from a mantle region more like (QFM). The ilmenitic xenoliths subjected to IOF analysis in our laboratory have exhibited auto-oxidation and therefore are difficult to interpret unequivocally. The high-confidence IOF data do argue strongly for mantle redox inhomogeneity, but much more work is needed to establish whether there is a general systematic redox decrease with depth into the mantle, on which the observed redox inhomogeneity is superimposed.