Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220, 1,200, and 1,180 degrees C), and at three different oxygen fugacities (fO(2) = IW, QFM, air) in order to assess the effect of fO(2) on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS. Most trace element Ds increase slightly as temperature decreases, except for D(Zr), D(Fe), D (Eu) and D(Cr) that vary systematically with fO(2). Applying the Lattice Strain Model to our data suggests the presence of Fe(2+)entirely in the octahedral site at highly to moderate reducing conditions, while Fe(3+) was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO(2) and confirm the greater compatibility of Eu(2+) which has an ionic radius similar to Sr, relative to Eu(3+) in plagioclase and the higher Eu(2+) / Eu(3+) under reducing conditions. For petrogenetic basaltic processes, a combined fractionation of Eu(2+) and Fe-Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks.1536647667