The oxygen fugacities (fO2s) recorded by rocks from the Earth's upper mantle have been the subject of much recent study and controversy1-7. Discussion has been stimulated by reported differences of several orders of magnitude between measurements made by different methods (Fig. 1). Oxygen fugacity is an important parameter because, together with temperature, pressure and composition, fO2 controls the petrogenesis of mantle-derived magmas, affects the composition and speciation of mantle fluids, and is an initial input into geochemical models of the evolution of the Earths crust, mantle and hydrosphere. Here we report the determination of the fO2 recorded by upper mantle spinel-lherzolite xenoliths entrained in alkaline magmas. This was done by experimentally calibrating the activity of the Fe3O4 (magnetite) component in MgAl2O4-rich synthetic spinel and applying these data to calculate thermobarometric fO2s for the three-phase assemblage livine-orthopyroxene-spinel. Results for appropriate model xenolith compositions, corrected to ISkbar total pressure 8 and plotted in temperature-fO2 space, fall at or above the synthetic quartz-fayalite-magnetite buffer (Fig. 3). In contrast to earlier studies2,3, we conclude that the shallow upper mantle does not retain an fO2 signature of equilibrium with the metallic core, and that gaseous species in the C-H-O system will be dominated by CO2 and H2O (refs 9, 10), rather than CH4 and H2.