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Abstract NASA's Genesis Mission returned solar wind (SW) to the Earth for analysis to derive the composition of the solar photosphere from solar material.SWanalyses control the precision of the derived solar compositions, but their ultimate accuracy is limited by the theoretical or empirical models of fractionation due toSWformation. Mg isotopes are “ground truth” for these models since, except forCAIs, planetary materials have a uniform Mg isotopic composition (within ≤1‰) so any significant isotopic fractionation ofSWMg is primarily that ofSWformation and subsequent acceleration through the corona. This study analyzed Mg isotopes in a bulkSWdiamond‐like carbon (DLC) film on silicon collector returned by the Genesis Mission. A novel data reduction technique was required to account for variable ion yield and instrumental mass fractionation (IMF) in theDLC. The resultingSWMg fractionation relative to theDSM‐3 laboratory standard was (−14.4‰, −30.2‰) ± (4.1‰, 5.5‰), where the uncertainty is 2ơSEof the data combined with a 2.5‰ (total) error in theIMFdetermination. Two of theSWfractionation models considered generally agreed with our data. Their possible ramifications are discussed for O isotopes based on theCAInebular composition of McKeegan et al. (2011).