The partitioning of molybdenum between magnetite (mt) and two rhyolitic melts has been determined at 800 degrees C, 1 kb of pressure, and at two oxygen fugacities. At an oxygen fugacity one-half a log unit above nickel-nickel oxide, D (super mt/melt) Mo = 0.21 + or - 0.08. A decrease in the oxygen fugacity to the graphite-methane buffer is accompanied by an increase in D (super mt/melt) Mo to 0.52 + or - 0.13. The differences in melt composition have no discernible effect.These data are interpreted by writing balanced chemical potential relationships involving independently variable phase components. The Nernst partition coefficients given above are then formulated as a function of these equilibria. Given the observed variation in D (super mt/melt) Mo with oxygen fugacity, and assuming that Fe 2 MoO 4 is the molybdenum-bearing phase component in magnetite, our data suggest that Mo(III) may be present in naturally occurring silicic magmas. Further, we suggest the establishment of critical bulk partition coefficients for ore-forming systems, defined in terms of critical values of the efficiency of removal function defined by Candela and Holland (1986). These preliminary results indicate that the crystallization of ferromagnesian or titanium-bearing phases may be important in reducing the amount of molybdenum available for orthomagmatic-hydrothermal ore formation. Oxygen fugacity may play a large role in the melt-crystal-vapor equilibria ofmolybdenum under magmatic conditions.