The partitioning of Mg and Fe between olivine and mafic-ultramafic melts has been examined based on previous and new experimental data. We find that the exchange partition coefficient, KD, changes primarily as a function of melt composition, expressed either as MgO+FeO content or degree of polymerization, NBO/T, rather than temperature or pressure. KD increases from about 0.23¿0.02 at an NBO/T of ~0 to a maximum of 0.35¿0.02 at an NBO/T of ~2, and then decreases to 0.23¿0.02 for the pure olivine system (NBO/T=4). We suggest that the following empirical relation yields a reliable estimate of KD for mantle derived melts: KD=0.235+0.120⋅(NBO/T)-0.031⋅(NBO/T)2. The change of KD shown here should be taken into account when calculating the composition of olivine coexisting with basaltic to ultramafic melts. ¿ 1998 American Geophysical Union