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Abstract: Previous work has shown that Mo isotopes measurably fractionate between metal and silicate liquids, even at temperatures appropriate for core formation. However, the effect of variations in the structural environment of Mo in the silicate liquid, especially as a function of valence state, on Mo isotope fractionation remained poorly explored. We have investigated the role of valence state in metal-silicate experiments in a gas-controlled furnace at 1400 °C and at oxygen fugacities between 10−12.7 and 10–9.9, i.e. between three and 0.2 log units below the iron-wüstite buffer. Two sets of experiments were performed, both with a silicate liquid in the CaO-Al2O3-SiO2 system. One set used molybdenum metal wire loops as the metal source, the other liquid gold alloyed with 2.5 wt% Mo contained in silica glass tubes. X-ray absorption near-edge spectroscopy analysis indicates that Mo6+/ΣMo in the silicate glasses varies between 0.24 and 0.77 at oxygen fugacities of 10–12.0 and 10–9.9 in the wire loop experiments and between 0.15 and 0.48 at 10–11.4 and 10–9.9 in the experiments with Au-Mo alloys. Double-spiked analysis of Mo isotope compositions furthermore shows that Mo isotope fractionation between metal and silicate is a linear function of Mo6+/ΣMo in the silicate glasses, with a difference of 0.51‰ in 98Mo/95Mo between purely Mo4+-bearing and purely Mo6+-bearing silicate liquid. The former is octahedrally and the latter tetrahedrally coordinated. Our study implies that previous experimental work contained a mixture of Mo4+ and Mo6+ species in the silicate liquid. Our refined parameterisation for Mo isotope fractionation between metal and silicate can be described as Δ98/95Mometal–silicate=−1.43±0.14×106Mo6+/ΣMo+8±6×104T2 Molybdenum isotope ratios therefore have potential as a proxy to constrain the oxygen fugacity during core formation on planetary bodies if the parameterisation of Mo6+/ΣMo variation with oxygen fugacity is expanded, for instance to include iron-bearing systems. On Earth literature data indicate that the upper mantle is depleted in heavy Mo isotopes relative to the bulk Earth, as represented by chondrites. As previously highlighted, this difference is most likely not caused by core formation, which either enriches the mantle in heavy Mo isotopes or causes no significant fractionation, depending on temperature and, as we determined here, Mo6+ content. We reaffirm that core formation does not account for the Mo isotope composition of the modern upper mantle, which may instead reflect the effect of fractionation during subduction as part of global plate recycling.