57Fe Mössbauer spectroscopy is employed to estimate Fe3+ ΣFe ratios and the Fe2O3 content of 28 well characterized pyroxenite and peridotite xenoliths from central France, southeastern Australia, northern Tanzania, southern Africa and Siberia. The Fe2O3 contents of various mantle regions were assessed, and found to range from 0.1 to 0.4 wt%. With an appraisal of the likely mantle inventories of C, H and S, the results demonstrate that these volatile elements and Fe all have similar buffering capacities for oxygen in peridotites. For this reason, the oxidation states of upper mantle peridotites should not generally be controlled by any particular buffer system. New estimates for the oxidation states of cratonic garnet peridotites indicate that the upper mantle beneath cratons is within 1 log unit of the fayalite-magnetite-quartz (FMQ) buffer, but may have been progressively oxidized since its original formation in the early Archean.