The Fe-Mg exchange coefficient between olivine (ol) and melt (m), defined as Kd Fe T −Mg = (Fe ol /Fe m)•(Mg m /Mg ol), with all Fe T expressed as Fe 2+ , is one of the most widely used parameters in petrology. We explore the effect of redox conditions on Kd Fe T −Mg using experimental, olivine-saturated basaltic glasses with variable H 2 O (≤ 7 wt%) over a wide range of fO 2 (ironwüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025-1425 °C) and melt composition. The ratio of Fe 3+ to total Fe (Fe 3+ /∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0-0.84. Measured Fe 3+ /∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H 2 O. Combining our new data with published experimental data having measured glass Fe 3+ /∑Fe, we show that for Fo 65-98 olivine in equilibrium with basaltic and basaltic andesite melts, Kd Fe T −Mg decreases linearly with Fe 3+ /∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo 50 olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe 3+ /∑Fe in olivine-bearing experimental melts, from which fO 2 can be calculated. In contrast to Kd Fe T −Mg , the Mn-Mg exchange coefficient, Kd Mn−Mg , is relatively constant over a wide range of P-T-fO 2 conditions. We present an expression for Kd Mn−Mg that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for Kd Fe T −Mg and Kd Mn−Mg to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe 3+ /∑Fe to a precision of ≤ 0.04.