We have determined DEu between augite and melt in samples that crystallized from a highly spiked martian basalt composition at four fO2 conditions. DEu augite/melt shows a steady increase with fO2 from 0.086 at IW−1 to 0.274 at IW+3.5. This increase is because Eu3+ is more compatible than Eu2+ in the pyroxene structure; thus increasing fO2 leads to greater Eu3+/Eu2+ in the melt and more Eu (total) can partition into the crystallizing pyroxene. This interpretation is supported by direct determinations of Eu valence state by XANES, which show a steady increase of Eu3+/Eu2+ with increasing fO2 in both pyroxene (0.38 to 14.6) and glass (0.20 to 12.6) in the samples. Also, pyroxene Eu3+/Eu2+ is higher than that of adjacent glass in all the samples, which verifies that Eu3+ is more compatible than Eu2+ in the pyroxene structure. Combining partitioning data with XANES data allows for the calculation of specific valence state D-values for augite/melt where DEu3+ = 0.28 and DEu2+ = 0.07.